[Ada] Create internal type for null array aggregate as an itype

[Ada] Create internal type for null array aggregate as an itype

Internal type created for the null array aggregate of Ada 2022 was
created as a temporary entity and then flagged as internal, but it is
better to create this type directly as an itype.

In particular, when the null array aggregate appears in a spec
expression, its type declaration will not be attached to the AST.
An itype will have Associated_Node_For_Itype, so that the context of
the type can be recovered, which is what GNATprove does.

gcc/ada/

	* sem_aggr.adb (Resolve_Null_Array_Aggregate): Create internal
	type for the aggregate as an itype.