[Ada] Implement calls to abstract subprograms in class-wide pre/post-conditions

[Ada] Implement calls to abstract subprograms in class-wide pre/post-conditions

In some special cases involving class-wide pre/post conditions, Ada
allows a non-dispatching call to an abstract function (which is usually
illegal).  Fix a bug in the implementation of Ada's rules about the
run-time behavior of such a call. Thanks to Javier Miranda for producing
this patch.

gcc/ada/

	* contracts.adb (Build_Call_Helper_Body): Improve handling of
	the case of a (legal) non-dispatching call to an abstract
	subprogram.