[Ada] Improve error message on array aggregates

[Pierre-Marie de Rodat via Gcc-patches]

Contrary to record aggregates, array aggregate cannot mix named and
positional associations. Make it clear in the error message.

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

gcc/ada/

        * sem_aggr.adb (Resolve_Array_Aggregate): Improve error message.
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
@@ -1954,7 +1954,7 @@ package body Sem_Aggr is
                    or else (Nb_Choices = 1 and then not Others_Present))
       then
          Error_Msg_N
-           ("named association cannot follow positional association",
+           ("cannot mix named and positional associations in array aggregate",
             First (Choice_List (First (Component_Associations (N)))));
          return Failure;
       end if;