[Ada] Wording problems with predicates (AI12-0099)

[Ada] Wording problems with predicates (AI12-0099)

gcc/ada/

	* sem_aux.adb: Add a with clause for Nlists.
	(Nearest_Ancestor): Test for the case of concurrent
	types (testing for both Is_Concurrent_Type and
	Is_Concurrent_Record_Type), and return the first ancestor in the
	Interfaces list if present (otherwise will return Empty if no
	interfaces).
	* sem_ch13.adb (Build_Predicate_Functions): Add a ??? comment
	about missing handling for adding predicates when they can be
	inherited from multiple progenitors.