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

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-armv8-quick` running on `linaro-clang-armv8-quick` while building `clang` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/154/builds/14281

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
FAIL: test_diagnostic_category


```

</details>

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