coroutines, ensure placeholder var is properly declared.

coroutines, ensure placeholder var is properly declared.

In cases that we need to extended the lifetime of a temporary captured
by reference, we make a replacement var for the temporary.  This will
be then used to define a coroutine frame entry (so that the var created
is elided by a later phase).  However, we should ensure that the var
is correctly declared anyway.

gcc/cp/ChangeLog:

2020-04-07  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (maybe_promote_captured_temps): Ensure that
	reference capture placeholder vars are properly declared.