Strip interface tags from visible fields

Strip interface tags from visible fields

The following Ada type:

   type Circle is new Shape and Drawable with record
        Center : Point;
        Radius : Natural;
   end record;

...is displayed as follow in GDB:

 (gdb) ptype circle
 type = new classes.shape with record
     V51s: ada.tags.interface_tag;
     center: classes.point;
     radius: natural;
 end record

V51s is an internal field that is of no interest for the user. It should
not be displayed.

gdb/ChangeLog:

	* ada-lang.c (ada_is_interface_tag): New function.
	(ada_is_ignored_field): Add interface tags to the list
	of ignored fields.