implement and document a new __has_feature and __has_builtin magic

builtin preprocessor macro.  This appears to work with two caveats:
1) builtins are registered in -E mode, and 2) target-specific builtins
are unconditionally registered even if they aren't supported by the
target (e.g. SSE4 builtin when only SSE1 is enabled).

llvm-svn: 73289