[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-with-lto-ubuntu` running on `as-worker-91` while building `clang` at step 7 "test-stage1-compiler".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/49/builds/1238

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

```
Step 7 (test-stage1-compiler) failure: build (failure)
0.067 [1/2/1] Preparing lit tests
0.632 [1/1/2] cd /home/buildbot/as-worker-91/clang-with-lto-ubuntu/llvm-project/clang/bindings/python && /usr/bin/cmake -E env CLANG_NO_DEFAULT_CONFIG=1 CLANG_LIBRARY_PATH=/home/buildbot/as-worker-91/clang-with-lto-ubuntu/build/stage1/lib /usr/bin/python3.10 -m unittest discover
FAILED: tools/clang/bindings/python/tests/CMakeFiles/check-clang-python /home/buildbot/as-worker-91/clang-with-lto-ubuntu/build/stage1/tools/clang/bindings/python/tests/CMakeFiles/check-clang-python 
cd /home/buildbot/as-worker-91/clang-with-lto-ubuntu/llvm-project/clang/bindings/python && /usr/bin/cmake -E env CLANG_NO_DEFAULT_CONFIG=1 CLANG_LIBRARY_PATH=/home/buildbot/as-worker-91/clang-with-lto-ubuntu/build/stage1/lib /usr/bin/python3.10 -m unittest discover
........................................................................F.....F..............................................................................
======================================================================
FAIL: test_diagnostic_category (tests.cindex.test_diagnostics.TestDiagnostics)
Ensure that category properties work.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/as-worker-91/clang-with-lto-ubuntu/llvm-project/clang/bindings/python/tests/cindex/test_diagnostics.py", line 82, in test_diagnostic_category
    self.assertEqual(d.category_number, 2)
AssertionError: 3 != 2

======================================================================
FAIL: test_diagnostic_string_format (tests.cindex.test_diagnostics.TestDiagnostics)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/as-worker-91/clang-with-lto-ubuntu/llvm-project/clang/bindings/python/tests/cindex/test_diagnostics.py", line 124, in test_diagnostic_string_format
    self.assertEqual(
AssertionError: "t.c:1:26: error: expected ';' after struct [2, Parse Issue]" != "t.c:1:26: error: expected ';' after struct [3, Parse Issue]"
- t.c:1:26: error: expected ';' after struct [2, Parse Issue]
?                                             ^
+ t.c:1:26: error: expected ';' after struct [3, Parse Issue]
?                                             ^


----------------------------------------------------------------------
Ran 157 tests in 0.516s

FAILED (failures=2)
ninja: build stopped: subcommand failed.

```

</details>

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