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

[Chuanqi Xu via cfe-commits]

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@@ -339,6 +339,9 @@ Bug Fixes in This Version
 - Fixes an assertion failure on invalid code when trying to define member
   functions in lambdas.
 
+- Clang now emits errors for coroutine `await_suspend` functions whose return type is not
+  one of `void`, `bool`, or `std::coroutine_handle`.
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ChuanqiXu9 wrote:

Maybe we should move this to `Potential Breaking Section` and mention it conforms the standard wording.


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