c++, coroutines: Use decltype(auto) for the g_r_o.

The revised wording for coroutines, uses decltype(auto) for the
type of the get return object, which preserves references.

It is quite reasonable for a  coroutine body implementation to
complete before control is returned to the ramp - and in that
case we would be creating the ramp return object from an already-
deleted promise object.

Jason observes that this is a terrible situation and we should
seek a resolution to it via core.

Since the test added here explicitly performs the unsafe action
dscribed above we expect it to fail (until a resolution is found).

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::build_ramp_function): Use
	decltype(auto) to determine the type of the temporary
	get_return_object.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr115908.C: Count promise construction
	and destruction. Run the test and XFAIL it.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
(cherry picked from commit e71a6e002c6650a7a7be99277120d3e59ecb78a3)