[clang] [clang-tools-extra] [compiler-rt] [Clang] [NFC] Fix more `-Wreturn-type` warnings in tests everywhere (PR #123470)

[Aaron Ballman via cfe-commits]

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

LGTM; I'm sure plenty of these could have changed the return type to `void`, but I think adding a `return` statement is fine as well (it means we don't have to question whether the return type mattered elsewhere in the tests).

https://github.com/llvm/llvm-project/pull/123470