[clang] [NFC] add DiagnosticsTestHelper decl (PR #112820)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

Fix whatever clang-format hates about this, and please add a detailed comment explaining why we are doing this, else LGTM.

https://github.com/llvm/llvm-project/pull/112820