ada: Crash in prefix notation with access to class-wide object

The compiler crashes analyzing a prefix notation call when its
prefix is an access to a class-wide object, an actual parameter
is missing, and the sources are compiled with language extensions
(-gnatX) and full errors (-gnatf).

gcc/ada/ChangeLog:

	* sem_ch4.adb (Try_Object_Operation): if no candidate interpretation
	matches the context, redo the same analysis with Report_Error True
	to report the error.