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

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yicuixi wrote:

> 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.

Done

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