[Ada] Ada_2020: Implement Key_Expression for named container aggregates

[Ada] Ada_2020: Implement Key_Expression for named container aggregates

gcc/ada/

	* par-ch4.adb: (P_Aggregate_Or_Paren_Expr): Recognize
	Iterated_Element_Component.
	(P_Iterated_Component_Association): Rebuild node as an Iterated_
	Element_Association when Key_Expression is present, and attach
	either the Loop_Parameter_Specification or the
	Iterator_Specification to the new node.
	* sem_aggr.adb: (Resolve_Container_Aggregate):
	Resolve_Iterated_Association handles bota Iterated_Component_
	and Iterated_Element_Associations, in which case it analyzes and
	resoles the orresponding Key_Expression.
	* exp_aggr.adb (Expand_Iterated_Component): If a Key_Expression
	is present, use it as the required parameter in the call to the
	insertion routine for the destination container aggregate. Call
	this routine for both kinds of Iterated_Associations.