ada: Avoid iterator conflicts in container aggregates

Create temporary scope for the iterators defined in a
container aggregate so that it would not be put to the
same scope where the expression was used. This would
otherwise lead to multiple aggregates with iterators that have
the same name leading to a name conflict.

gcc/ada/

	* sem_aggr.adb (Resolve_Iterated_Association): Add temporary scope
	when analyzing the Iterator Specification. Use preanalysis instead
	of Analysis to avoid polluting the tree.