ASTContext::mergeTypes(): Loosen up the type checking for 'Class' (treating it like 'id').

This fixes <rdar://problem/6782722> XCDataTipsManager.m registers, observes notifications in class methods.

The radar above is the result of clang typing 'self' in a class method as 'Class', which results in some spurious warnings (GCC types 'self' in a class method as 'id').

I considered changing the type of 'self' to 'id' (to conform to GCC), however this resulted in *many* test cases breaking. In addition, I really prefer a more strongly typed 'self'.

All in all, this is the least obtrusive fix I could find for removing the spurious warnings (though we do loose some valid warnings).

llvm-svn: 69041