Problem reported as bug #230184 also happens with devel/llvm60. And though I don't checked it is very likely this problem happens with other version of LLVM.
I don't expect this to work. You almost certainly need to specify the path to libstdc++'s headers manually.
So far as I know llvm does not provide the source to a
libstdc++. Some port would be required to supply the
source code for -stdlib=libstdc++ to find and use.
(For FreeBSD "builworld" and such there are licensing
issues for libstdc++. Thus a port likely being involved.)
I won't be fixing this. To fix it reliably, we'd have to provide a dependency on a gcc port to provide a libstdc++ in a known location and patch the -stdlib=libstdc++ code to match. If want libstdc++, use gcc.