[Ada] Missing predicate check with explicit policy pragma

[Arnaud Charlet]

This patch fixes an omission in the generation of predicate checks, when there
is a configuration pragma that sets Assertion_Policy to Ignore and a subsequent
configuration pragma that enables predicate checking.

Executing:

   gnatmake -q main
   main

must yield

raised SYSTEM.ASSERTIONS.ASSERT_FAILURE :
        Dynamic_Predicate failed at main.adb:5

---
pragma Assertion_Policy (Ignore);
 pragma Assertion_Policy
       (Static_Predicate => Check,
       Dynamic_Predicate => Check,
       Pre => Check, Pre'Class => Check);
---
with Pred;
procedure Main is
begin
  Pred.Foo ("");
end;
---
package Pred is
  subtype Not_Empty_String is String
    with Dynamic_Predicate => Not_Empty_String /= "";
  procedure Foo (S : Not_Empty_String);
end;
---
package body Pred is
  procedure Foo (S : Not_Empty_String) is begin null; end;
end;

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

2016-10-12  Ed Schonberg  <schonb...@adacore.com>

        * sem_prag.adb (Analyze_Pragma, case Dynamic_Predicate):
        Check properly whether there is an explicit assertion policy
        for predicate checking, even in the presence of a general Ignore
        assertion policy.


Index: sem_prag.adb
===================================================================
--- sem_prag.adb        (revision 241048)
+++ sem_prag.adb        (working copy)
@@ -19136,15 +19136,17 @@
             --  the rep item chain, for processing when the type is frozen.
             --  This is accomplished by a call to Rep_Item_Too_Late. We also
             --  mark the type as having predicates.
-            --  If the current policy is Ignore mark the subtype accordingly.
-            --  In the case of predicates we consider them enabled unless an
-            --  Ignore is specified, to preserve existing warnings.
 
+            --  If the current policy for predicate checking is Ignore mark the
+            --  subtype accordingly. In the case of predicates we consider them
+            --  enabled unless Ignore is specified (either directly or with a
+            --  general Assertion_Policy pragma) to preserve existing warnings.
+
             Set_Has_Predicates (Typ);
             Set_Predicates_Ignored (Typ,
               Present (Check_Policy_List)
                 and then
-                  Policy_In_Effect (Name_Assertion_Policy) = Name_Ignore);
+                  Policy_In_Effect (Name_Dynamic_Predicate) = Name_Ignore);
             Discard := Rep_Item_Too_Late (Typ, N, FOnly => True);
          end Predicate;