[clang] Surface error for plain return statement in coroutine earlier (PR #100985)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

Thanks! The changes LG, I'll wait until premerge checks finish and merge it into mainline.

https://github.com/llvm/llvm-project/pull/100985