coroutines: Add a helper for creating local vars.

coroutines: Add a helper for creating local vars.

This is primarily code factoring, but we take this opportunity
to rename some of the implementation variables (which we intend
to expose to debugging) so that they are in the implementation
namespace.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/cp/ChangeLog:

	* coroutines.cc (coro_build_artificial_var): New.
	(build_actor_fn): Use var builder, rename vars to use
	implementation namespace.
	(coro_rewrite_function_body): Likewise.
	(morph_fn_to_coro): Likewise.

(cherry picked from commit a45a7ecdf34311587daa2e90cc732adcefac447b)