Simplify C++ type-printing

The C++ type-printing code had its own variant of the accessibility
enum.  This patch removes this and changes the code to use the new one
from gdbtypes.h.

This patch also changes the C++ code to recognize the default
accessibility of a class.  This makes ptype a bit more C++-like, and
lets us remove a chunk of questionable code.

Acked-By: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Keith Seitz <keiths@redhat.com>