[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.