ada: Set Is_Local_Anonymous_Access on the anonymous access type of return objects

It is already set on the anonymous access type of function results and will
prevent return objects from being treated as Ada 2012's stand-alone objects,
thus make it easier to enforce the RM 3.10.2(13.2) rule.

gcc/ada/ChangeLog:

	* einfo.ads (Is_Local_Anonymous_Access): Mention return objects.
	* accessibility.adb (Type_Access_Level): Call Enclosing_Subprogram.
	* sem_ch3.adb (Find_Type_Of_Object): Set Is_Local_Anonymous_Access
	on the anonymous access type of return objects.