coroutines: Make proxy vars for the function arg copies.

This adds top level proxy variables for the coroutine frame
copies of the original function args.  These are then available
in the debugger to refer to the frame copies.  We rewrite the
function body to use the copies, since the original parms will
no longer be in scope when the coroutine is running.

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

gcc/cp/ChangeLog:

	* coroutines.cc (struct param_info): Add copy_var.
	(build_actor_fn): Use simplified param references.
	(register_param_uses): Likewise.
	(rewrite_param_uses): Likewise.
	(analyze_fn_parms): New function.
	(coro_rewrite_function_body): Add proxies for the fn
	parameters to the outer bind scope of the rewritten code.
	(morph_fn_to_coro): Use simplified version of param ref.