[Ada] Ada_2020 AI12-0250 : Implement Iterator filters.

[Ada] Ada_2020 AI12-0250 : Implement Iterator filters.

gcc/ada/

	* par.adb (P_Iterator_Specification): Make public for use in
	other parser subprograms.
	* par-ch4.adb (P_Iterated_Component_Association): In Ada_2020,
	recognize use of Iterator_Specification in an element iterator.
	To simplify disambiguation between the two iterator forms, mark
	the component association as carrying an Iterator_Specification
	only when the element iterator (using "OF") is used.
	* par-ch5.adb (P_Loop_Parameter_Specification): In Ada_2020,
	parse iterator filter when present.
	(P_Iterator_Specification): Ditto.  Remove declaration of
	P_Iterator_Specification, now in parent unit.
	* exp_ch5.adb (Expand_N_Loop_Statement): Apply Iterator filter
	when present.
	(Expand_Iterator_Loop_Over_Array): Ditto.
	(Expand_Iterator_Loop_Over_Container): Ditto.
	* sem_aggr.adb (Resolve_Array_Aggregate): Emit error nessage if
	an iterated component association includes a iterator
	specificcation with an element iterator, i.e. one that uses the
	OF keyword.
	* sem_ch5.adb (Analyze_Iterator_Specification): Analyze Iterator
	filter when present.
	(Analyze_Loop_Parameter_Specification): Ditto.
	* sinfo.adb: Suprogram bodies for new syntactic element
	Iterator_Filter.
	* sinfo.ads: Add Iterator_Filter to relevant nodes.  Structure
	of Component_Association and Iteroted_Component_Association
	nodes is modified to take into account the possible  presence of
	an iterator specification in the latter.