[Ada] Crash on discriminant check with current instance

[Ada] Crash on discriminant check with current instance

gcc/ada/

	* checks.adb (Build_Discriminant_Checks): Add condition to
	replace references to the current instance of the type when we
	are within an Init_Proc.
	(Replace_Current_Instance): Examine a given node and replace the
	current instance of the type with the corresponding _init
	formal.
	(Search_And_Replace_Current_Instance): Traverse proc which calls
	Replace_Current_Instance in order to replace all references
	within a given expression.