[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.
diff --git a/gcc/ada/sem_aggr.adb b/gcc/ada/sem_aggr.adb
--- a/gcc/ada/sem_aggr.adb
+++ b/gcc/ada/sem_aggr.adb
@@ -1837,7 +1837,7 @@ package body Sem_Aggr is
       if Others_Present and then not Others_Allowed then
          Error_Msg_N
            ("OTHERS choice not allowed here",
-            First (Choices (First (Component_Associations (N)))));
+            First (Choice_List (First (Component_Associations (N)))));
          return Failure;
       end if;