[Ada] Reject overlays in Global/Depends/Initializes contracts

[Ada] Reject overlays in Global/Depends/Initializes contracts

gcc/ada/

	* sem_prag.adb (Analyze_Depends_In_Decl_Part): Reject overlays
	in Depends and Refined_Depends contracts.
	(Analyze_Global_In_Decl_Part): Likewise for Global and
	Refined_Global.
	(Analyze_Initializes_In_Decl_Part): Likewise for Initializes
	(when appearing both as a single item and as a initialization
	clause).
	* sem_util.ads (Ultimate_Overlaid_Entity): New routine.
	* sem_util.adb (Report_Unused_Body_States): Ignore overlays.
	(Ultimate_Overlaid_Entity): New routine.