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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov requested changes to this pull request.

The `assert` condition seems wrong, otherwise this should be good to go.

I've also left one more suggestion to leave a comment that I missed in the original review since we need another round anyway.

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