[Ada] Simplify data structures for overloaded interpretations

[Ada] Simplify data structures for overloaded interpretations

gcc/ada/

	* sem_type.ads (Write_Interp_Ref): Removed; no longer needed.
	* sem_type.adb (Headers): Removed; now the hash table is
	directly in the Interp_Map alone.
	(Interp_Map): Now an instance of the GNAT.HTable.Simple_HTable.
	(Last_Overloaded): New variable to emulate Interp_Map.Last.
	(Add_One_Interp): Adapt to new data structure.
	(Get_First_Interp): Likewise.
	(Hash): Likewise.
	(Init_Interp_Tables): Likewise.
	(New_Interps): Likewise.
	(Save_Interps): Likewise; handle O_N variable like in
	Get_First_Interp.
	(Write_Interp_Ref): Removed; no longer needed.