[Ada] Complain about missing -gnat2020 switch on iterator_filter

[Pierre-Marie de Rodat]

Reuse Error_Msg_Ada_2020_Feature to complain about a missing -gnat2020
switch when compiling iterator_filter that would be legal in Ada 2020,
but the compiler is expecting an earlier version of the language.

A small enhancement; opportunity spotted while adding support for
iterator filters in GNATprove.

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

gcc/ada/

        * par-ch5.adb (P_Loop_Parameter_Specification): Complain about
        missing -gnat2020 switch.
        (P_Iterator_Specification): Likewise.
diff --git a/gcc/ada/par-ch5.adb b/gcc/ada/par-ch5.adb
--- a/gcc/ada/par-ch5.adb
+++ b/gcc/ada/par-ch5.adb
@@ -1712,9 +1712,9 @@ package body Ch5 is
       Set_Discrete_Subtype_Definition
         (Loop_Param_Specification_Node, P_Discrete_Subtype_Definition);
 
-      if Ada_Version >= Ada_2020
-         and then Token = Tok_When
-      then
+      if Token = Tok_When then
+         Error_Msg_Ada_2020_Feature ("iterator filter", Token_Ptr);
+
          Scan; -- past WHEN
          Set_Iterator_Filter
            (Loop_Param_Specification_Node, P_Condition);
@@ -1773,9 +1773,9 @@ package body Ch5 is
 
       Set_Name (Node1, P_Name);
 
-      if Ada_Version >= Ada_2020
-         and then Token = Tok_When
-      then
+      if Token = Tok_When then
+         Error_Msg_Ada_2020_Feature ("iterator filter", Token_Ptr);
+
          Scan; -- past WHEN
          Set_Iterator_Filter
            (Node1, P_Condition);