Introduce TargetInfo::hasFeature() to query various feature names in

each of the targets. Use this for module requirements, so that we can
pin the availability of certain modules to certain target features,
e.g., provide a module for xmmintrin.h only when SSE support is
available.

Use these feature names to provide a nearly-complete module map for
Clang's built-in headers. Only mm_alloc.h and unwind.h are missing,
and those two are fairly specialized at the moment. Finishes
<rdar://problem/10710060>.

llvm-svn: 149227