c++, coroutines: Allow convertible get_return_on_allocation_fail [PR109682].

c++, coroutines: Allow convertible get_return_on_allocation_fail [PR109682].

We have been requiring the get_return_on_allocation_fail() call to have the
same type as the ramp.  This is not intended by the standard, so relax that
to allow anything convertible to the ramp return.

	PR c++/109682

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::build_ramp_function): Allow for cases where
	get_return_on_allocation_fail has a type convertible to the ramp
	return type.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr109682.C: New test.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
(cherry picked from commit f4915e6c4cd42e7d6f397dc36fab507cc47dad05)