c++, coroutines: Only allow void get_return_object if the ramp is void [PR100476].

c++, coroutines: Only allow void get_return_object if the ramp is void [PR100476].

Require that the value returned by get_return_object is convertible to
the ramp return.  This means that the only time we allow a void
get_return_object, is when the ramp is also a void function.

We diagnose this early to allow us to exit the ramp build if the return
values are incompatible.

	PR c++/100476

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::build_ramp_function): Remove special
	handling of void get_return_object expressions.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/coro-bad-gro-01-void-gro-non-class-coro.C:
	Adjust expected diagnostic.
	* g++.dg/coroutines/pr102489.C: Avoid void get_return_object.
	* g++.dg/coroutines/pr103868.C: Likewise.
	* g++.dg/coroutines/pr94879-folly-1.C: Likewise.
	* g++.dg/coroutines/pr94883-folly-2.C: Likewise.
	* g++.dg/coroutines/pr96749-2.C: Likewise.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
(cherry picked from commit a0b431033c307982123abbff752045cfe7eda47f)