Remove byte vectors from cplus_struct_type

This removes some byte vectors from cplus_struct_type, moving the
information into bitfields in holes in struct field.

A new 'enum accessibility' is added to hold some of this information.
A similar enum is removed from c-varobj.c.

Note that the stabs reader treats "ignored" as an accessibility.
However, the stabs texinfo documents this as a public field that is
optimized out -- unfortunately nobody has updated the stabs reader to
use the better value-based optimized-out machinery.  I looked and
apparently gcc never emitted this visibility value, so whatever
compiler generated this stab is unknown.  I left a comment in
gdbtypes.h to this effect.

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