ada: Refine types for an accessibility-checking routine

Code cleanup related to work on expression functions for GNATprove
(which require accessibility checks even when they are not expanded
and thus have no explicit return statements).

gcc/ada/

	* accessibility.adb
	(Is_Formal_Of_Current_Function): This routine expects an entity
	reference and not the entity itself, so its parameter is a Node_Id
	and not an Entity_Id.