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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

Thanks for the change!
This looks like an improvement, but I suspect we can do even better. See the comments I've left in the code.

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