I repeatedly failed to perform a buildworld using the latest 10.3Beta from svn due to 
c++: error: no such file or directory: '/usr/obj/prod/100202/P/PRESCOTT/i386.i386/pd3/src/tmp/pd3/src/usr.bin/clang/tblgen/../../../lib/clang/libllvmtablegen/libllvmtablegen.a'
c++: error: no such file or directory: '/usr/obj/prod/100202/P/PRESCOTT/i386.i386/pd3/src/tmp/pd3/src/usr.bin/clang/tblgen/../../../lib/clang/libllvmsupport/libllvmsupport.a'

I commented out SUBDIR_PARALLEL= in src/lib/clang/Makefile which was successful but it made the build horrendously slow. Re-ordering the sequence fixed the problem and returned to a speedy build.

--- /pd3/src/lib/clang/Makefile.orig    2014-05-27 18:19:16.000000000 +1000
+++ /pd3/src/lib/clang/Makefile 2016-02-05 17:14:19.000000000 +1100
@@ -36,6 +40,9 @@
        libclangserialization \
        ${_libclangstaticanalyzer} \
        \
+       libllvmsupport \
+       libllvmtablegen \
+       \
        libllvmanalysis \
        libllvmasmparser \
        libllvmasmprinter \
@@ -56,8 +63,6 @@
        libllvmoption \
        libllvmscalaropts \
        libllvmselectiondag \
-       libllvmsupport \
-       libllvmtablegen \
        libllvmtarget \
        libllvmtransformutils \
        libllvmvectorize \

Yes, libllvmtablegen also caused a problem.  One build job was, also, trying to use it before it was built...