coroutines: Factor code. Match original source location in helpers [NFC].

coroutines: Factor code. Match original source location in helpers [NFC].

This is primarily a source code refactoring, the only change is to
ensure that the outlined functions are marked to begin at the same
line as the original.  Otherwise, they get the default (which seems
to be input_location, which corresponds to the closing brace at the
point that this is done).  Having the source location point to that
confuses some debuggers.

This is a contributory fix to:
PR c++/99215 - coroutines: debugging with gdb

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

	* coroutines.cc (build_actor_fn): Move common code to
	act_des_fn.
	(build_destroy_fn): Likewise.
	(act_des_fn): Build the void return here.  Ensure that the
	source location matches the original function.