coroutines: Collect the function body rewrite code.

The standard describes a rewrite of the body of the user-authored
function (which wraps it in a try-catch block and provides the
initial and final suspend expressions).  The exact arrangement of
this was still in flux right up until the DIS and as a consequence
was a bit of a moving target.

The net result was a fragmented implementation of the parts of
the rewrite which is now impeding progress in fixing other issues.

This patch collates the rewrite action into a single function and
carries this out earlier.

gcc/cp/ChangeLog:

	* coroutines.cc (expand_one_await_expression): Remove
	code dealing with initial suspend.
	(build_actor_fn): Remove code special-casing initial
	and final suspend. Handle the final suspend and marking
	of the coroutine as done.
	(coro_rewrite_function_body): New.
	(bind_expr_find_in_subtree): Remove.
	(coro_body_contains_bind_expr_p): Remove.
	(morph_fn_to_coro): Split the rewrite of the original
	function into coro_rewrite_function_body and call it.

(cherry picked from commit 9252a208f485eed2757d601657facfa0aee6cd21)