[Ada] Ada_2020: ongoing work for aggregates for bounded containers

[Ada] Ada_2020: ongoing work for aggregates for bounded containers

gcc/ada/

	* sem_aggr.adb: (Resolve_Container_Aggregate): For an indexed
	container, verify that expressions and component associations
	are not both present.
	* exp_aggr.adb: Code reorganization, additional comments.
	(Expand_Container_Aggregate): Use Aggregate_Size for Iterated_
	Component_Associations for indexed aggregates. If present, the
	default value of the formal in the constructor function is used
	when the size of the aggregate cannot be determined statically.