coroutines: Add cleanups, where required, to statements with captured references.

coroutines: Add cleanups, where required, to statements with captured references.

When we promote captured temporaries to local variables, we also
remove their initializers from the relevant call expression.  This
means that we should recompute the need for a cleanup expression
once the set of temporaries that remains becomes known.

gcc/cp/ChangeLog:

2020-04-08  Iain Sandoe  <iain@sandoe.co.uk>
	    Jun Ma  <JunMa@linux.alibaba.com>

	* coroutines.cc (maybe_promote_captured_temps): Add a
	cleanup expression, if needed, to any call from which
	we promoted temporaries captured by reference.