c++/coro: prevent ICV_STATEMENT diagnostics in temp promotion [PR116502]

c++/coro: prevent ICV_STATEMENT diagnostics in temp promotion [PR116502]

If such a diagnostic is necessary, it has already been emitted,
otherwise, it is not correct and emitting it here is inactionable by the
user, and bogus.

	PR c++/116502

gcc/cp/ChangeLog:

	* coroutines.cc (maybe_promote_temps): Convert temporary
	initializers to void without complaining.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/maybe-unused-1.C: New test.
	* g++.dg/coroutines/pr116502.C: New test.