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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

I still feel it would be simpler to lookup the function for `await_suspend` in the awaiter and look for its return type. Then we don't need to care about the type of the expression.

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