c++, coroutines: Handle allocation fail returns [PR121219].

c++, coroutines: Handle allocation fail returns [PR121219].

The current implementation was returning the result of the g_r_o_o_a_f
call independently of the return expressions for 'normal' cases.

This prevents the NVRO that we need to guarantee copy elision for the
ramp return values - when these are initialised from a temporary of the
same type.

The solution here reorders the code so that the regular return expression
appears before the allocation-failed case.  Ensure that the g_r_o and
associated code appears in a distinct scope.  These steps are to meet the
constaints of NRV.

	PR c++/121219

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::build_ramp_function): Reorder the return
	expressions for the 'normal' and 'allocation failed' cases so that
	NRV constraints are met.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/torture/pr121219.C: New test.

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