[Ada] Improve error message when named number passed as global item

[Pierre-Marie de Rodat <derodat at adacore dot com>]

When a named number is used in Global/Depends contracts as global item,
the error message could be confusing to users new to Ada. Now the
message explains that named numbers are not objects.

For instance on the following illegal code snippet:

     $ gcc -c bad_global.ads

     1. package Bad_Global is
     2.
     3.    X : constant := 1;
     4.    Y : constant := 1.0;
     5.
     6.    procedure P with
     7.      Global => (Input => X,
                                 |
        >>> global item must denote object, state or current instance
            of concurrent type
        >>> named number "X" is not an object

     8.                 In_Out => Y);
                                  |
        >>> global item must denote object, state or current instance
            of concurrent type
        >>> named number "Y" is not an object

     9.
    10. end Bad_Global;

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

2018-12-11  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Global_Item): Refine error message.

--- gcc/ada/sem_prag.adb
+++ gcc/ada/sem_prag.adb
@@ -2282,6 +2282,12 @@ package body Sem_Prag is
                   SPARK_Msg_N
                     ("global item must denote object, state or current "
                      & "instance of concurrent type", Item);
+
+                  if Ekind (Item_Id) in Named_Kind then
+                     SPARK_Msg_NE
+                       ("\named number & is not an object", Item, Item);
+                  end if;
+
                   return;
                end if;