[Ada] Assertion_Policy for class-wide precondition

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.

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

gcc/ada/

	* exp_disp.adb (Build_Class_Wide_Check): Return early if the
	precondition is supposed to be ignored.

From-SVN: r262789

gcc/ada/ChangeLog

+2018-07-17  Bob Duff  <duff@adacore.com>
+
+	* exp_disp.adb (Build_Class_Wide_Check): Return early if the
+	precondition is supposed to be ignored.
+
 2018-07-17  Ed Schonberg  <schonberg@adacore.com>
 
 	* sem_ch6.adb (Check_Untagged_Equality): Extend check to operations

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;