ada: Skip dynamic interface conversion under native runtime

ada: Skip dynamic interface conversion under native runtime

gcc/ada/

	* exp_disp.adb
	(Has_Dispatching_Constructor_Call): New subprogram.
	(Expand_Interface_Conversion): No need to perform dynamic
	interface conversion when the operand and the target type are
	interface types and the target interface type is an ancestor of
	the operand type. The unique exception to this rule is when the
	operand has a dispatching constructor call (as documented in the
	sources).