[Ada] Detect expansion of iterated component associations into loops

[Ada] Detect expansion of iterated component associations into loops

Iterated component associations are expanded into loops, which GNAT
should detect as violating restriction No_Implicit_Loops; same for
iterated element associations and delta array aggregates.

Part of cleanups for correct handling of iterated component associations
in SPARK.

gcc/ada/

	* exp_aggr.adb
	(Two_Pass_Aggregate_Expansion): Expand into implicit rather than
	ordinary loops, to detect violations of restriction
	No_Implicit_Loops.
	(Generate_Loop): Likewise for delta array aggregates.