MFC r275477:

Pull in r174303 from upstream gcc trunk (by Jason Merrill):

  PR c++/48211
  * name-lookup.h (cp_class_binding): Make base a pointer.
  * name-lookup.c (new_class_binding): Adjust.
  (poplevel_class): Adjust.

This fixes a potential segfault when compiling gold, a part of the
devel/binutils port, with gcc.  See also the upstream bug report:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48211

Thanks to Jason Merrill, Tom Callaway and Red Hat legal for approving
the use of this patch under the GNU GPL, version 2 or later.