[clang] [coroutines] Introduce [[clang::coro_return_type]] and [[clang::coro_wrapper]] (PR #71945)

[Haojian Wu via cfe-commits]

https://github.com/hokein commented:

Thanks, the change looks good from my side. I will leave the final stamp to @ChuanqiXu9 and @ilya-biryukov.

https://github.com/llvm/llvm-project/pull/71945