[clang] [Clang] [NFC] Introduce a helper for emitting compatibility diagnostics (PR #132348)

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Sirraide wrote:

Ah, I think I’ve found what the problem is: it would seem that moving the diagnostics around in the file caused whatever the ‘diagnostic category number’ is supposed to be of some of these diagnostics to change, and there were tests that check for that that still need to be updated. I’ll update the test and then try to reland this.

https://github.com/llvm/llvm-project/pull/132348