ada: Compiler crash on container aggregate with loop_parameter_specifications

ada: Compiler crash on container aggregate with loop_parameter_specifications

The compiler crashes on a container aggregate with more than one
iterated_element_association given by a loop_parameter_specification.
In such a case, the tree contains N_Iterated_Component_Association
nodes rather than N_Iterated_Element_Association nodes, and the code
for handling those needs to obtain the bounds from the Discrete_Choices
field of each N_Iterated_Component_Association rather than assuming
that the association has a normal list of choices.

gcc/ada/

	* sem_aggr.adb (Resolve_Container_Aggregate): In the case where Comp
	is an N_Iterated_Component_Association, pick up Discrete_Choices rather
	than Choices.