c++, coroutines: Improve check for throwing final await [PR104051].

We check that the final_suspend () method returns a sane type (i.e. a class
or structure) but, unfortunately, that check has to be later than the one
for a throwing case.  If the use returns some nonsensical type from the
method, we need to handle that in the checking for noexcept.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

	PR c++/104051

gcc/cp/ChangeLog:

	* coroutines.cc (coro_diagnose_throwing_final_aw_expr): Handle
	non-target expression inputs.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr104051.C: New test.