ada: Expose utility routine for processing of Depends contracts in SPARK

ada: Expose utility routine for processing of Depends contracts in SPARK

Routine Is_Unconstrained_Or_Tagged_Item is now used both in the GNAT
frontend (for checking legality of Depends clauses) and in the GNATprove
backend (for representing implicit inputs in flow graphs).

gcc/ada/

	* sem_prag.adb (Is_Unconstrained_Or_Tagged_Item): Move to
	Sem_Util, so it can be used from GNATprove.
	* sem_util.ads (Is_Unconstrained_Or_Tagged_Item): Move from
	Sem_Prag; spec.
	* sem_util.adb (Is_Unconstrained_Or_Tagged_Item): Move from
	Sem_Prag; body.