[Ada] Allow uninitialized values on Big_Positive/Natural

[Pierre-Marie de Rodat]

The current Dynamic_Predicate prevents declaring a variable without also
initializing it at the same time.

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

2020-06-15  Arnaud Charlet  <char...@adacore.com>

gcc/ada/

        * libgnat/a-nbnbin.ads (Big_Positive, Big_Natural): Fix
        predicate.
--- gcc/ada/libgnat/a-nbnbin.ads
+++ gcc/ada/libgnat/a-nbnbin.ads
@@ -46,11 +46,15 @@ is
    function To_Big_Integer (Arg : Integer) return Big_Integer;
 
    subtype Big_Positive is Big_Integer
-     with Dynamic_Predicate => Big_Positive > To_Big_Integer (0),
+     with Dynamic_Predicate =>
+            (if Is_Valid (Big_Positive)
+             then Big_Positive > To_Big_Integer (0)),
           Predicate_Failure => (raise Constraint_Error);
 
    subtype Big_Natural is Big_Integer
-     with Dynamic_Predicate => Big_Natural >= To_Big_Integer (0),
+     with Dynamic_Predicate =>
+            (if Is_Valid (Big_Natural)
+             then Big_Natural >= To_Big_Integer (0)),
           Predicate_Failure => (raise Constraint_Error);
 
    function In_Range (Arg, Low, High : Big_Integer) return Boolean is