ada: Constraint_Error caused by interface conversion

When the sources have a type conversion from an interface type
T2 to some ancestor interface type T1 (that is, T2 extends T1)
the tag check added by the compiler may fail at runtime.

gcc/ada/

	* exp_disp.adb (Has_Dispatching_Constructor_Call): Removed.
	(Expand_Interface_Conversion): Reverse patch.