coroutines: Implicitly movable objects should use move CTORs for co_return.

coroutines: Implicitly movable objects should use move CTORs for co_return.

This is a case where the standard contains conflicting information.
after discussion between implementators, the accepted intent is of
[class.copy.elision].  This amends the handling of co_return statements
to follow that.

gcc/cp/ChangeLog:

2020-05-16  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (finish_co_return_stmt): Implement rules
	from [class.copy.elision] /3.

gcc/testsuite/ChangeLog:

2020-05-16  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/co-return-syntax-10-movable.C: New test.