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

gcc/ada/

	* exp_aggr.adb (Build_Siz_Exp): new function, subsidiary of
	Expand_Container_Aggregate, to create an expression to be used
	in the dynamic allocation of a container with a single container
	element association.
	(Add_Range): Handle static bounds of ranges over enumerations.
	(Expand_Container_Aggregate): Add declaration for size
	expression when needed, and use it in container object
	declaration for container.