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

[via cfe-commits]

Sirraide wrote:

> How about
> 
> * in this patch you modify the tests, and the test runner (hopefully lit config can do that, I haven't checked) such that ~all tests (including new ones) use -Werror=return-type (we might want to have tests where it's just a warning to make sure this still works)
> * In a separate patch we flip the default.

Yeah, I agree that that makes sense (I was thinking about splitting it too since there are so many effectively NFC changes that are part of this...); I’ve changed this pr to just refactor all the tests. I don’t expect too many new tests to be added between when this is merged and when the actual change is made to make the warning an error since the vast majority of the tests (at least of the ones that I spent the most time refactoring) are the old ones (from around 2007–2010).

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