objc_gc wants a pointer type, not a function type; give it a more appropriate

objc_gc wants a pointer type, not a function type;  give it a more appropriate
diagnostic.  Also, these attributes are commonly written with macros which we
actually pre-define, so instead of expanding the macro location, refer to the
instantiation location and name it using the macro loc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127219 91177308-0d34-0410-b5e6-96231b3b80d8