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

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Sirraide wrote:

> anything you can do to reduce white space only changes?

Er, unless there is some way to do it w/ git that I’m not aware of it’d be pretty complicated for me because my IDE deletes trailing whitespace on save (and I’m pretty sure that’s true for a lot of other people too), so it becomes fairly cumbersome to try and edit a file w/o doing that—that and I am so used to it doing that that I’m pretty sure e.g. disabling that setting would have the opposite effect and introduce a bunch of trailing whitespace changes in every file I edit... :(

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