[Ada] Fix handling of access-to-variable objects in Global and Depends

gcc/ada/

	* sem_prag.ads (Collect_Subprogram_Inputs_Outputs): Update
	comment; this routine is no longer used by GNATprove.
	* sem_prag.adb (Find_Role): The IN parameter is on output only
	when it belongs to non-function; also, the otherwise constant
	object can only be written by a non-function.
	(Collect_Global_Item): The IN parameter can only be written when
	it belongs to non-function; also, unnest this check to make it
	easier to read.