c++, coroutines: Fix handling of bool await_suspend() [PR115905].

c++, coroutines: Fix handling of bool await_suspend() [PR115905].

As noted in the PR the action of the existing implementation was to
treat a false value from await_suspend () as equivalent to "do not
suspend".  Actually it needs to be the equivalent of "resume" - and
we need to restart the dispatcher - since the await_suspend() body
could have already resumed the coroutine.
See also https://github.com/cplusplus/CWG/issues/601 (NAD) for more
discussion.

Since we need to amend the await expansion and the actor build, take
the opportunity to clean up and modernise the code there.  Note that
we need to make the jump back to the dispatcher without any scope
exit cleanups (so we have to use the .CO_SUSPN IFN to do this).

	PR c++/115905

gcc/cp/ChangeLog:

	* coroutines.cc (struct coro_aw_data): Add a member for the
	restart dispatch label.
	(expand_one_await_expression): Rework to modernise and to
	handle the boolean await_suspend() case.
	(build_actor_fn): Rework the dispatcher and allow for a jump
	back to the dispatcher.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/torture/pr115905.C: New test.

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