[Ada] Implement AI12-0280's interactions with container aggregates

[Ada] Implement AI12-0280's interactions with container aggregates

gcc/ada/

	* sem_util.adb (Is_Container_Aggregate): A new local predicates
	which indicates whether a given expression is a container
	aggregate. The implementation of this function is incomplete; in
	the unusual case of a record aggregate (i.e., not a container
	aggregate) of a type whose Aggregate aspect is specified, the
	function will incorrectly return True.
	(Immediate_Context_Implies_Is_Potentially_Unevaluated): Improve
	handling of aggregate components.
	(Is_Repeatedly_Evaluated): Test for container aggregate
	components along with existing test for array aggregate
	components.