c++, coroutines: Rework the ramp codegen.

c++, coroutines: Rework the ramp codegen.

Now that we have separated the codegen of the ramp, actor and
destroy functions, we no longer need to manage the scopes for
variables manually.

This introduces a helper function that allows us to build a
local var with a DECL_VALUE_EXPR that relates to the coroutine
state frame entry.

This fixes a latent issue where we would generate guard vars
when exceptions were disabled.

gcc/cp/ChangeLog:

	* coroutines.cc (coro_build_artificial_var_with_dve): New.
	(coro_build_and_push_artificial_var): New.
	(coro_build_and_push_artificial_var_with_dve): New.
	(analyze_fn_parms): Ensure that frame entries cannot clash
	with local variables.
	(build_coroutine_frame_delete_expr): Amend comment.
	(cp_coroutine_transform::build_ramp_function): Rework to
	avoid manual management of variables and scopes.

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