coroutines: Update handling and failure for g-r-o-o-a-f [PR95505]

coroutines: Update handling and failure for g-r-o-o-a-f [PR95505]

The actual issue is that (in the testcase) std::nothrow is not
available.  So update the handling of the get-return-on-alloc-fail
to include the possibility that std::nothrow might not be
available.

gcc/cp/ChangeLog:

	PR c++/95505
	* coroutines.cc (morph_fn_to_coro): Update handling of
	get-return-object-on-allocation-fail and diagnose missing
	std::nothrow.

gcc/testsuite/ChangeLog:

	PR c++/95505
	* g++.dg/coroutines/pr95505.C: New test.