ada: Fix Ultimate_Overlaid_Entity to match the SPARK RM semantics

ada: Fix Ultimate_Overlaid_Entity to match the SPARK RM semantics

The Ultimate_Overlaid_Entity function should return the root of the
last precisely supported address clause as per the definition in the
SPARK RM.

gcc/ada/ChangeLog:

	* sem_util.ads (Overlaid_Entity): Return the root of the address
	clause of an object if it is precisely supported in SPARK.
	* sem_util.adb (Ultimate_Overlaid_Entity): Use Overlaid_Entity to
	match the SPARK RM semantics.
	* sem_prag.adb (Analyze_Global_Item): Only check for overlays on
	variables.
	(Analyze_Initialization_Item): Likewise.
	(Analyze_Input_Item): Likewise.