Add support for the MS qualifiers __ptr32, __ptr64, __sptr, __uptr.

Summary:
This adds parsing of the qualifiers __ptr32, __ptr64, __sptr, and __uptr and
lowers them to the corresponding address space pointer for 32-bit and 64-bit pointers.
(32/64-bit pointers added in https://reviews.llvm.org/D69639)

A large part of this patch is making these pointers ignore the address space
when doing things like overloading and casting.

https://bugs.llvm.org/show_bug.cgi?id=42359

Reviewers: rnk, rsmith

Subscribers: jholewinski, jvesely, nhaehnle, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71039