Modify __has_attribute so that it only looks for GNU-style attributes. Removes...

Modify __has_attribute so that it only looks for GNU-style attributes. Removes the ability to look for generic attributes and keywords via this macro, which has the potential to be a breaking change. However, since there is __has_cpp_attribute and __has_declspec_attribute, and given the limited usefulness of querying a generic attribute name regardless of syntax, this seems like the correct path forward.

llvm-svn: 223468