[Ada] Ongoing work for AI12-0212 : container aggregates

[Ada] Ongoing work for AI12-0212 : container aggregates

gcc/ada/

	* sem_aggr.adb (Resolve_Container_Aggregate): Add semantic
	checks for indexed aggregates, including component associations
	and iterated component associations.
	* exp_aggr.adb (Expand_Iterated_Component): New subprogram,
	subsidiary of Expand_Container_Aggreggate, used for positional,
	named, and indexed aggregates.
	(Aggregate_Size): New subprogram to precompute the size of an
	indexed aggregate prior to call to allocate it.
	(Expand_Range_Component): New subprogram so generate loop for a
	component association given by a range or a subtype name in an
	indexed aggregate.