[clang] [clang] Accept empty enum in MSVC compatible C (PR #159981)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Can you please explain the problem in the summary instead of just pointing to an issue. The summary should explain basics required to understand the PR w/o having to jump to the issue right away for context.

So, What are you changing, why are you changing it and how.

https://github.com/llvm/llvm-project/pull/159981