Add support for embedding Clang compiler headers
like tgmath.h and stdarg.h into the LLDB installation,
and then finding them through the Host infrastructure.

Also add a script to actually do this on Mac OS X.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@223430 91177308-0d34-0410-b5e6-96231b3b80d8