I have a Dell laptop 5470 8GB of RAM and custom kernel updated via svn FreeBSD 11.2-RELEASE-p4 #1 r339175M: Wed Oct 3 23:26:44 -03 2018

Updated portsnap:

$ sudo portsnap fetch update

$ cd /usr/ports/lang/swift/

$ sudo make install
===>  Building for swift-3.1.1_7
cd /usr/ports/lang/swift/work/swift-3.1.1; /usr/bin/env PERL_USE_UNSAFE_INC=1 XDG_DATA_HOME=/usr/ports/lang/swift/work  XDG_CONFIG_HOME=/usr/ports/lang/swift/work  HOME=/usr/ports/lang/swift/work PATH=/usr/ports/lang/swift/work/.bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin:/home/test/bin NO_PIE=yes MK_DEBUG_FILES=no MK_KERNEL_SYMBOLS=no SHELL=/bin/sh NO_LINT=YES DESTDIR=/usr/ports/lang/swift/work/stage LANG=en_US.UTF-8 LC_ALL=en_US.UTF-8 PREFIX=/usr/local  LOCALBASE=/usr/local  CC="cc" CFLAGS="-O2 -pipe  -DLIBICONV_PLUG -fstack-protector -isystem /usr/local/include -fno-strict-aliasing "  CPP="cpp" CPPFLAGS="-DLIBICONV_PLUG -isystem /usr/local/include"  LDFLAGS=" -lobjc -fstack-protector -L/usr/local/lib " LIBS=""  CXX="c++" CXXFLAGS="-O2 -pipe -DLIBICONV_PLUG -fstack-protector -isystem /usr/local/include -fno-strict-aliasing  -DLIBICONV_PLUG -isystem /usr/local/include "  MANPREFIX="/usr/local" BSD_INSTALL_PROGRAM="install  -s -m 555"  BSD_INSTALL_LIB="install  -s -m 0644"  BSD_INSTALL_SCRIPT="install  -m 555"  BSD_INSTALL_DATA="install  -m 0644"  BSD_INSTALL_MAN="install  -m 444"  ./swift/utils/build-script --preset=mixin_freebsd_installation  -j 4  install_destdir=/usr/ports/lang/swift/work/stage  --    --skip-build-llvm=1  --install_prefix=/usr/local  --install_destdir=/usr/ports/lang/swift/work/stage  --verbose-build=1  --install-llbuild=1  --skip-test-llbuild=1
./swift/utils/build-script: note: using preset 'mixin_freebsd_installation', which expands to 

./swift/utils/build-script --assertions --no-swift-stdlib-assertions --llbuild --release -- --swift-enable-ast-verifier=0 --install-swift --install-lldb --install-llbuild --install-xctest --install-prefix=/usr/local '--swift-install-components=autolink-driver;compiler;clang-builtin-headers;stdlib;swift-remote-mirror;sdk-overlay;license;tools;editor-integration' --skip-build-libdispatch --build-swift-static-stdlib --build-swift-static-sdk-overlay --build-swift-stdlib-unittest-extra --install-destdir=/usr/ports/lang/swift/work/stage --jobs 4

Building the standard library for: swift-stdlib-freebsd-x86_64
cmark: using gold linker
ninja: no work to do.
llvm: using gold linker
symlinking the system headers (/usr/include/c++) into the local clang build directory (/usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/llvm-freebsd-x86_64/include).
ninja: no work to do.
swift: using gold linker
[1/590] Building CXX object lib/AST/CMakeFiles/swiftAST.dir/TypeWalker.cpp.o
[2/590] Building CXX object lib/AST/CMakeFiles/swiftAST.dir/Stmt.cpp.o
FAILED: lib/AST/CMakeFiles/swiftAST.dir/Stmt.cpp.o 
/usr/bin/clang++  -DGTEST_HAS_RTTI=0 -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Ilib/AST -I/usr/ports/lang/swift/work/swift-3.1.1/swift/lib/AST -I/usr/ports/lang/swift/work/swift-3.1.1/swift/include -Iinclude -I/usr/ports/lang/swift/work/swift-3.1.1/llvm/include -I/usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/llvm-freebsd-x86_64/include -I/usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/llvm-freebsd-x86_64/tools/clang/include -I/usr/ports/lang/swift/work/swift-3.1.1/llvm/tools/clang/include -I/usr/ports/lang/swift/work/swift-3.1.1/cmark/src -I/usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/cmark-freebsd-x86_64/src -fno-stack-protector -fPIC -fvisibility-inlines-hidden -Wall -W -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wcovered-switch-default -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Werror=date-time -std=c++11 -fcolor-diagnostics -ffunction-sections -fdata-sections -Wdocumentation -Wimplicit-fallthrough -Wunreachable-code -Woverloaded-virtual -O3    -UNDEBUG  -fno-exceptions -fno-rtti -target x86_64-unknown-freebsd11.2-RELEASE-p4 -O2 -momit-leaf-frame-pointer -g0 -UNDEBUG -MD -MT lib/AST/CMakeFiles/swiftAST.dir/Stmt.cpp.o -MF lib/AST/CMakeFiles/swiftAST.dir/Stmt.cpp.o.d -o lib/AST/CMakeFiles/swiftAST.dir/Stmt.cpp.o -c /usr/ports/lang/swift/work/swift-3.1.1/swift/lib/AST/Stmt.cpp
In file included from /usr/ports/lang/swift/work/swift-3.1.1/swift/lib/AST/Stmt.cpp:17:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/Stmt.h:20:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/Availability.h:20:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/Type.h:20:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/llvm/include/llvm/ADT/DenseMap.h:17:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/llvm/include/llvm/ADT/DenseMapInfo.h:17:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/llvm/include/llvm/ADT/ArrayRef.h:13:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/llvm/include/llvm/ADT/Hashing.h:48:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/llvm-freebsd-x86_64/include/llvm/Support/DataTypes.h:33:
In file included from /usr/include/c++/v1/cmath:305:
In file included from /usr/include/c++/v1/math.h:309:
/usr/include/c++/v1/type_traits:1366:38: error: incomplete type 'swift::Expr' used in type trait expression
    : public integral_constant<bool, __is_abstract(_Tp)> {};
                                     ^
/usr/include/c++/v1/type_traits:1456:39: note: in instantiation of template class 'std::__1::is_abstract<swift::Expr>' requested here
                                     !is_abstract<_T2>::value> {};
                                      ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/Basic/NullablePtr.h:40:22: note: in instantiation of template class 'std::__1::is_convertible<swift::Expr, swift::Expr>' requested here
                std::is_convertible<OtherT, T>::value,
                     ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/Basic/NullablePtr.h:30:7: note: while substituting deduced template arguments into function template 'NullablePtr' [with OtherT = swift::Expr]
class NullablePtr {
      ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/Stmt.h:772:7: note: while declaring the implicit copy constructor for 'ForStmt'
class ForStmt : public LabeledStmt {
      ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/TypeAlignments.h:35:9: note: forward declaration of 'swift::Expr'
  class Expr;
        ^
1 error generated.
[3/590] Building CXX object lib/AST/CMakeFiles/swiftAST.dir/TypeRefinementContext.cpp.o
FAILED: lib/AST/CMakeFiles/swiftAST.dir/TypeRefinementContext.cpp.o 
/usr/bin/clang++  -DGTEST_HAS_RTTI=0 -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Ilib/AST -I/usr/ports/lang/swift/work/swift-3.1.1/swift/lib/AST -I/usr/ports/lang/swift/work/swift-3.1.1/swift/include -Iinclude -I/usr/ports/lang/swift/work/swift-3.1.1/llvm/include -I/usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/llvm-freebsd-x86_64/include -I/usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/llvm-freebsd-x86_64/tools/clang/include -I/usr/ports/lang/swift/work/swift-3.1.1/llvm/tools/clang/include -I/usr/ports/lang/swift/work/swift-3.1.1/cmark/src -I/usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/cmark-freebsd-x86_64/src -fno-stack-protector -fPIC -fvisibility-inlines-hidden -Wall -W -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wcovered-switch-default -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Werror=date-time -std=c++11 -fcolor-diagnostics -ffunction-sections -fdata-sections -Wdocumentation -Wimplicit-fallthrough -Wunreachable-code -Woverloaded-virtual -O3    -UNDEBUG  -fno-exceptions -fno-rtti -target x86_64-unknown-freebsd11.2-RELEASE-p4 -O2 -momit-leaf-frame-pointer -g0 -UNDEBUG -MD -MT lib/AST/CMakeFiles/swiftAST.dir/TypeRefinementContext.cpp.o -MF lib/AST/CMakeFiles/swiftAST.dir/TypeRefinementContext.cpp.o.d -o lib/AST/CMakeFiles/swiftAST.dir/TypeRefinementContext.cpp.o -c /usr/ports/lang/swift/work/swift-3.1.1/swift/lib/AST/TypeRefinementContext.cpp
In file included from /usr/ports/lang/swift/work/swift-3.1.1/swift/lib/AST/TypeRefinementContext.cpp:17:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/ASTContext.h:20:
In file included from /usr/ports/lang/swift/work/swift-3.1.1/build/Ninja-ReleaseAssert+stdlib-Release/llvm-freebsd-x86_64/include/llvm/Support/DataTypes.h:33:
In file included from /usr/include/c++/v1/cmath:305:
In file included from /usr/include/c++/v1/math.h:309:
/usr/include/c++/v1/type_traits:1366:38: error: incomplete type 'swift::Expr' used in type trait expression
    : public integral_constant<bool, __is_abstract(_Tp)> {};
                                     ^
/usr/include/c++/v1/type_traits:1456:39: note: in instantiation of template class 'std::__1::is_abstract<swift::Expr>' requested here
                                     !is_abstract<_T2>::value> {};
                                      ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/Basic/NullablePtr.h:40:22: note: in instantiation of template class 'std::__1::is_convertible<swift::Expr, swift::Expr>' requested here
                std::is_convertible<OtherT, T>::value,
                     ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/Basic/NullablePtr.h:30:7: note: while substituting deduced template arguments into function template 'NullablePtr' [with OtherT = swift::Expr]
class NullablePtr {
      ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/Stmt.h:772:7: note: while declaring the implicit copy constructor for 'ForStmt'
class ForStmt : public LabeledStmt {
      ^
/usr/ports/lang/swift/work/swift-3.1.1/swift/include/swift/AST/TypeAlignments.h:35:9: note: forward declaration of 'swift::Expr'
  class Expr;
        ^
1 error generated.
[4/590] Building CXX object lib/AST/CMakeFiles/swiftAST.dir/TypeRepr.cpp.o
[5/590] Building CXX object lib/AST/CMakeFiles/swiftAST.dir/USRGeneration.cpp.o
ninja: build stopped: subcommand failed.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/lang/swift
*** Error code 1

Stop.
make: stopped in /usr/ports/lang/swift

the build is broken and does not generate the binary package.