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

[Ilya Biryukov via cfe-commits]

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

Thanks, I have a few small NIT (but feel free to ignore any number of them).

I do have a major question about the use of `getCurFunction()->isCoroutine()`, see the corresponding comment.

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