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

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

gcc/ada/

	* par-ch4.adb (P_Iterated_Component_Association): Extended to
	recognzize the similar Iterated_Element_Association. This node
	is only generated when an explicit Key_Expression is given.
	Otherwise the distinction between the two iterated forms is done
	during semantic analysis.
	* sinfo.ads: New node N_Iterated_Element_Association, for
	Ada202x container aggregates.  New field Key_Expression.
	* sinfo.adb: Subprograms for new node and newn field.
	* sem_aggr.adb (Resolve_Iterated_Component_Association): Handle
	the case where the Iteration_Scheme is an
	Iterator_Specification.
	* exp_aggr.adb (Wxpand_Iterated_Component): Handle a component
	with an Iterated_Component_Association, generate proper loop
	using given Iterator_Specification.
	* exp_util.adb (Insert_Axtions): Handle new node as other
	aggregate components.
	* sem.adb, sprint.adb: Handle new node.
	* tbuild.adb (Make_Implicit_Loop_Statement): Handle properly a
	loop with an Iterator_ specification.