c++/coroutines: Fix awaiter var creation [PR116506]

c++/coroutines: Fix awaiter var creation [PR116506]

Awaiters always need to have a coroutine state frame copy since
they persist across potential supensions.  It simplifies the later
analysis considerably to assign these early which we do when
building co_await expressions.

The cleanups in r15-3146-g47dbd69b1, unfortunately elided some of
processing used to cater for cases where the var created from an
xvalue, or is a pointer/reference type.

Corrected thus.

	PR c++/116506
	PR c++/116880

gcc/cp/ChangeLog:

	* coroutines.cc (build_co_await): Ensure that xvalues are
	materialised.  Handle references/pointer values in awaiter
	access expressions.
	(is_stable_lvalue): New.
	* decl.cc (cxx_maybe_build_cleanup): Handle null arg.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr116506.C: New test.
	* g++.dg/coroutines/pr116880.C: New test.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
Co-authored-by: Jason Merrill <jason@redhat.com>