[Sema] Split special builtin type lookups into a separate function

In case further such cases appear in the future we've got a generic function to add them to.
Additionally changed the ObjC special case to check the language and the identifier builtin ID instead of the name.

Addresses the cleanup suggestion from D87917.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D87983