[Ada] Assertion_Policy for class-wide precondition

[Pierre-Marie de Rodat]

This patch fixes the compiler to that class-wide preconditions on primitive
operations of interfaces are not checked at run time when the Assertion_Policy
indicates that they should be ignored. This is required by the RM.

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

2018-07-17  Bob Duff  <d...@adacore.com>

gcc/ada/

        * exp_disp.adb (Build_Class_Wide_Check): Return early if the
        precondition is supposed to be ignored.
--- gcc/ada/exp_disp.adb
+++ gcc/ada/exp_disp.adb
@@ -809,7 +809,7 @@ package body Exp_Disp is
                Prec := Next_Pragma (Prec);
             end loop;
 
-            if No (Prec) then
+            if No (Prec) or else Is_Ignored (Prec) then
                return;
             end if;