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

[via cfe-commits]

Sirraide wrote:

> LLVM Buildbot has detected a new failure on builder `lldb-arm-ubuntu` running on `linaro-lldb-arm-ubuntu` while building `clang` at step 6 "test".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/18/builds/13872
> 
> Here is the relevant piece of the build log for the reference

This builder seems to be fine now.

https://github.com/llvm/llvm-project/pull/134043