Fix Ada catchpoint regression

The breakpoint C++-ification series introduced a regression for Ada
catchpoints.  Specifically, commit 2b5ab5b8 ("Convert base breakpoints
to vtable ops") caused these to start failing.  I didn't notice this
because testing Ada using a Linux distro compiler requires installing
the GNAT debuginfo, which I hadn't done.

This patch fixes the problem.  I'm checking it in.