[clang] [Clang] Warn about `[[noreturn]]` on coroutines (PR #127623)

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Sirraide wrote:

> I still think it makes more sense to diagnose this when the FunctionDecl is created rather than when we try and return from it.

Huh, weird, github went ahead and posted that comment anyway even though it was complaining about the review message being empty. This was supposed to be in reference to the SemaStmt changes.

https://github.com/llvm/llvm-project/pull/127623