[clang] [Clang] [NFC] Fix unintended `-Wreturn-type` warnings everywhere in the test suite (PR #123464)

[via cfe-commits]

Sirraide wrote:

> It seems a bit ambitious to do this at the end of the cycle (no time to react to feedback).

Yeah, on the one hand, we could wait until after the branch before we make this change, but on the other hand, doing it now might result in more feedback as to how disruptive defaulting the warning to an error would actually end up being?

https://github.com/llvm/llvm-project/pull/123464