[clang] [clang][Parser] Warn when the body of expansion statement is not a compound statement (PR #209229)

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Sirraide wrote:

We just discussed this during Aaron’s office hours today, and we haven’t been able to find a reason as to why this is specified to require a _compound-statement_ rather than just a _statement_, so we want to spend some time investigating whether it wouldn’t make more sense to update standard to say _statement_ instead.

https://github.com/llvm/llvm-project/pull/209229