c++, coroutines: Address CWG2563 return value init [PR119916].

c++, coroutines: Address CWG2563 return value init [PR119916].

This addresses the clarification that, when the get_return_object is of a
different type from the ramp return, any necessary conversions should be
performed on the return expression (so that they typically occur after the
function body has started execution).

	PR c++/119916

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::wrap_original_function_body): Do not
	initialise initial_await_resume_called here...
	(cp_coroutine_transform::build_ramp_function): ... but here.
	When the coroutine is not void, initialize a GRO object from
	promise.get_return_object().  Use this as the argument to the
	return expression.  Use a regular cleanup for the GRO, since
	it is ramp-local.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/torture/special-termination-00-sync-completion.C:
	Amend for CWG2563 expected behaviour.
	* g++.dg/coroutines/torture/special-termination-01-self-destruct.C:
	Likewise.
	* g++.dg/coroutines/torture/pr119916.C: New test.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>