[Ada] Assertion_Policy for class-wide precondition

[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