Reapply "[Clang] [NFC] Introduce a helper for emitting compatibility diagnostics (#132348)"#134043

Sirraide merged 2 commits intollvm:mainllvm/llvm-project:mainfrom

Sirraide:reapply-compat-warnings-helperSirraide/llvm-project:reapply-compat-warnings-helperCopy head branch name to clipboard