Extend or truncate __ptr32/__ptr64 pointers when dereferenced.

Summary:
A while ago I implemented the functionality to lower Microsoft __ptr32
and __ptr64 pointers, which are stored as 32-bit and 64-bit pointer
and are extended/truncated to the appropriate pointer size when
dereferenced.
This patch adds an addrspacecast to cast from the __ptr32/__ptr64
pointer to a default address space when dereferencing.

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

Reviewers: hans, arsenm, RKSimon

Subscribers: wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81517