[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 `clang-aarch64-sve-vls` running on `linaro-g3-02` while building `clang` at step 7 "ninja check 1".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/143/builds/6630
> 
> Here is the relevant piece of the build log for the reference

Er, this one also seems unrelated:
```console
ld.lld: error: cannot open lib/libllvm_gtest.a: No such file or directory
```

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