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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223468 91177308-0d34-0410-b5e6-96231b3b80d8