coroutines: Improve diagnostics for one allocator case.

coroutines: Improve diagnostics for one allocator case.

If the user provides operator new and that is noexcept, this
implies that it can fail with a null return.  At that point, we expect
to be able to call get_return_object_on_allocation_failure().

This diagnoses the case where such an operator new has been
provided, but the g-r-o-o-a-f is either missing or unusable.

gcc/cp/ChangeLog:

	* coroutines.cc (morph_fn_to_coro): Diagnose unavailable
	get_return_object_on_allocation_failure.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/coro-bad-grooaf-01-grooaf-expected.C: New test.