[Sema] Allow comparisons between different ms ptr size address space types.

We're currently using address spaces to implement __ptr32/__ptr64 attributes;
this patch fixes a bug where clang doesn't allow types with different pointer
size attributes to be compared.

Fixes https://bugs.llvm.org/show_bug.cgi?id=51889

Differential Revision: https://reviews.llvm.org/D110670