c++, coroutines: Account for overloaded promise return_value() [PR105301].

c++, coroutines: Account for overloaded promise return_value() [PR105301].

Whether it was intended or not, it is possible to define a coroutine promise
with multiple return_value() methods [which need not even have the same type].

We were not accounting for this possibility in the check to see whether both
return_value and return_void are specifier (which is prohibited by the
standard).  Fixed thus and provided an adjusted diagnostic for the case that
multiple return_value() methods are present.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
	PR c++/105301

gcc/cp/ChangeLog:

	* coroutines.cc (coro_promise_type_found_p): Account for possible
	mutliple overloads of the promise return_value() method.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr105301.C: New test.