Several cleanups:

- rename isObjCIdType/isObjCClassType -> isObjCIdStructType/isObjCClassStructType. The previous name didn't do what you would expect.
- add back isObjCIdType/isObjCClassType to do what you would expect. Not currently used, however many of the isObjCIdStructType/isObjCClassStructType clients could be converted over time.
- move static Sema function areComparableObjCInterfaces to ASTContext (renamed to areComparableObjCPointerTypes, since it now operates on pointer types).

llvm-svn: 64385