[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 `ppc64le-lld-multistage-test` running on `ppc64le-lld-multistage-test` while building `clang` at step 7 "test-build-stage1-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/168/builds/10420

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

```
Step 7 (test-build-stage1-unified-tree-check-all) failure: test (failure)
FAIL: test_diagnostic_category

Step 13 (test-build-stage2-unified-tree-check-all) failure: test (failure)
FAIL: test_diagnostic_category


```

</details>

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