[Ada] Better recovery for misplaced keyword in discriminant specification

[Arnaud Charlet]

This patch improves the handling of an error in a discriminant specification
for an access discriminant.

Compiling b.ads must yield:

   b.ads:3:43: "constant" must appear after "access"

---
package B is

    type Constant_Reference (D : constant access Integer) is null record
       with Implicit_Dereference => D;
end B;

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

2015-10-20  Ed Schonberg  <schonb...@adacore.com>

        * par-ch3.adb (P_Known_Discriminant_Part_Opt): Handle properly
        a misplaced "constant" keyword in a discriminant specification.


Index: par-ch3.adb
===================================================================
--- par-ch3.adb (revision 229049)
+++ par-ch3.adb (working copy)
@@ -3030,8 +3030,23 @@
                   Set_Discriminant_Type
                     (Specification_Node,
                      P_Access_Definition (Not_Null_Present));
+
+               --  Catch ouf-of-order keywords
+
+               elsif Token = Tok_Constant then
+                  Scan;
+
+                  if Token = Tok_Access then
+                     Error_Msg_SC ("CONSTANT must appear after ACCESS");
+                     Set_Discriminant_Type
+                       (Specification_Node,
+                        P_Access_Definition (Not_Null_Present));
+
+                  else
+                     Error_Msg_SC ("misplaced CONSTANT");
+                  end if;
+
                else
-
                   Set_Discriminant_Type
                     (Specification_Node, P_Subtype_Mark);
                   No_Constraint;