[clang] [Clang] Coroutines: Properly Check if `await_suspend` return type is a `std::coroutine_handle` (PR #85684)

[Yuxuan Chen via cfe-commits]

yuxuanchen1997 wrote:

> @yuxuanchen1997 are you still working on that?

This isn't blocking me now as it accepted more programs than what standard requires and it's up to us to decide how much effort should be invested and how much NFC can be tolerated to fix this nonstandard/unimportant "feature" (or bug).

https://github.com/llvm/llvm-project/pull/85684