coroutines: Emit error for invalid promise return types [PR97438].

coroutines: Emit error for invalid promise return types [PR97438].

At one stage, use cases were proposed for allowing the promise
type to contain both return_value and return_void.  That was
not accepted into C++20, so we should reject it as per the PR.

gcc/cp/ChangeLog:

	PR c++/97438
	* coroutines.cc (struct coroutine_info): Add a field to
	record that we emitted a promise type error.
	(coro_promise_type_found_p): Check for the case that the
	promise type contains both return_void and return_value.
	Emit an error if so, with information about the wrong
	type methods.

gcc/testsuite/ChangeLog:

	PR c++/97438
	* g++.dg/coroutines/pr97438.C: New test.