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

[via cfe-commits]

https://github.com/Sirraide commented:

It feels a bit weird to put a function that is only used in a test right in the middle of a header, doubly so considering that GCC is the only compiler that doesn’t like it if the only declaration of a function is a friend declaration...

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