[ms] Fix Microsoft compatibility handling of commas in nested macro expansions.

In Microsoft-compatibility mode, single commas from nested macro expansions
should not be considered as argument separators; we already emulated this by
marking them to be ignored. However, in MSVC's preprocessor, subsequent
expansions DO treat these commas as argument separators... so we now ignore
each comma at most once.

Includes a small unit test that validates we match MSVC's behavior as shown
in https://gcc.godbolt.org/z/y0twaq

Fixes PR43282

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69626