c++, coroutines: Some cleanups in build_actor.

c++, coroutines: Some cleanups in build_actor.

We were incorrectly guarding all the frame cleanups on the
basis of frame_needs_free (which is always set for the present
code-gen since we have no allocation elision).  The net result
being that the (incorrect) code was behaving as expected.

We built, but never used, a label for the frame destruction;
in practice it is never triggered independently of the promise
and argument copy destruction.

Finally there are a few instances where we had been building
expressions manually rather than using higher-level APIs.

gcc/cp/ChangeLog:

	* coroutines.cc (build_actor_fn): Remove an unused
	label, guard the frame deallocation correctly, use
	simpler APIs to build if and return statements.

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