[Ada] Reuse Is_Named_Number where possible

gcc/ada/

	* sem_attr.adb, sem_prag.adb: Use Is_Named_Number.

gcc/ada/sem_attr.adb

@@ -6636,7 +6636,7 @@ package body Sem_Attr is
          Check_E0;
 
          if not Is_Entity_Name (P)
-           or else Ekind (Entity (P)) not in Named_Kind
+           or else not Is_Named_Number (Entity (P))
          then
             Error_Attr_P ("prefix for % attribute must be named number");
 

gcc/ada/sem_prag.adb

@@ -2383,7 +2383,7 @@ package body Sem_Prag is
                     ("global item must denote object, state or current "
                      & "instance of concurrent type", Item);
 
-                  if Ekind (Item_Id) in Named_Kind then
+                  if Is_Named_Number (Item_Id) then
                      SPARK_Msg_NE
                        ("\named number & is not an object", Item, Item_Id);
                   end if;