coroutines: Fix missed ramp function return copy elision [PR95346].

coroutines: Fix missed ramp function return copy elision [PR95346].

Confusingly, "get_return_object ()" can do two things:
- Firstly it can provide the return object for the ramp function (as
  the name suggests).
- Secondly if the type of the ramp function is different from that
  of the get_return_object call, this is used as a single parameter
  to a CTOR for the ramp's return type.

In the first case we can rely on finish_return_stmt () to do the
necessary processing for copy elision.
In the second case, we should have passed a prvalue to the CTOR as
per the standard comment, but I had omitted the rvalue () call.  Fixed
thus.

gcc/cp/ChangeLog:

	PR c++/95346
	* coroutines.cc (morph_fn_to_coro): Ensure that the get-
	return-object is constructed correctly; When it is not the
	final return value, pass it to the CTOR of the return type
	as an rvalue, per the standard comment.

gcc/testsuite/ChangeLog:

	PR c++/95346
	* g++.dg/coroutines/pr95346.C: New test.

(cherry picked from commit 4f2d05ef0142d269964e165c14c6f7fe4bdfd5a3)