[clang] [Clang] Use Compat diagnostics for most extension/compatibility warnings (PR #216693)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Thank you for this! LGTM but there may be one nit to clean up (feel free to ignore if I'm wrong about it being an existing typo though).

https://github.com/llvm/llvm-project/pull/216693