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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

LGTM. Please wait for 3~5 days in case @ilya-biryukov has more comments.

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