[Diagnostics] Warn for comparison with string literals expanded from macro (PR44064)

Summary:
As noted in PR, we have a poor test coverage for this warning. I think macro support was just overlooked. GCC warns in these cases.
Clang missed a real bug in the code I am working with, GCC caught it.

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70624