[Ada] Fix crash on illegal OTHERS in iterated_component_association

[Pierre-Marie de Rodat]

A code for detecting illegal OTHERS clause is handles both
component_association and iterated_component_association, whose choice
list must be accessed with Choices and Discrete_Choices, respectively.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_aggr.adb (Resolve_Array_Aggregate): Use Choice_List, which
	internally calls either Choice or Discrete_Choices, depending on
	the context.
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