Add support for target-specific builtins, including detecting nonportability

of source code.  For example:

$ clang INPUTS/carbon_h.c -arch i386 -arch ppc
prints:
...
/usr/lib/gcc/i686-apple-darwin8/4.0.1/include/mmintrin.h:51:3: note: use of a target-specific builtin function, source is not 'portable'
  __builtin_ia32_emms ();
  ^

because carbon.h pulls in xmmintrin.h, and __builtin_ia32_emms isn't a builtin on ppc.

Though clang now supports target-specific builtins, the full table isn't implemented yet.

llvm-svn: 39328