[Ada] Fix inconsistent building of itypes for null array aggregates

[Ada] Fix inconsistent building of itypes for null array aggregates

To analyze Ada 2022 null array aggregates we introduced a dedicated
routine and bypassed the code for ordinary array aggregates. However,
the types for the array indexes created by this dedicated routine
differed from the types created for ordinary array aggregates, i.e.
itypes for null array aggregates were associated with the array subtype
declaration, while itypes for ordinary array aggregates were associated
with the aggregate itself. These differences cause trouble for various
routines in GNATprove.

This patch reduces the special handling of null array aggregates and
reuses the building of itypes for ordinary array aggregates.

gcc/ada/

	* sem_aggr.adb
	(Array_Aggr_Subtype): Bypass call to Collect_Aggr_Bound with
	dedicated code for null array aggregates.
	(Resolve_Array_Aggregate): Remove special handling of null array
	aggregates.
	(Resolve_Array_Aggregate): Create bounds, but let
	Array_Aggr_Subtype create itype entities.