If we see a get_return_object_on_allocation_failure in the
promise, we expect to be able to use it.  If this isn't
possible (because of some error in the declaration) then we
need to handle the erroneous return to allow following code
to complete.

gcc/cp/ChangeLog:

	* coroutines.cc (morph_fn_to_coro): Handle error
	returns in building g-r-o-o-a-f expressions.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/coro1-allocators.h (BAD_GROOAF_STATIC):
	New.
	* g++.dg/coroutines/coro-bad-grooaf-00-static.C: New test.

(cherry picked from commit e195c8045a1f3604f0f6b339e9ff731ebd9dcb0e)