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

[via llvm-commits]

Sirraide wrote:

> (it means we don't have to question whether the return type mattered elsewhere in the tests).

Yeah, that’s the main reason I opted for adding a `return` statement in a lot of cases.

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