coroutines: Ensure the ramp return object is checked (PR93443).

As the PR shows, there is a pathway through the code where the
no_warning value is not set, which corresponds to a missing check
of the ramp return when it was constructed from the 'get return
object'  Fixed by ensuring that the check of the return value is
carried out for both return cases.

gcc/cp/ChangeLog:

2020-01-27  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/93443
	* coroutines.cc (morph_fn_to_coro): Check the ramp return
	value when it is constructed from the 'get return object'.