gdbsupport: make filtered_iterator::operator* return the same thing as underlying...

gdbsupport: make filtered_iterator::operator* return the same thing as underlying iterator

This is the same idea as the previous patch, but for filtered_iterator.
Without this patch, I would see this when applying the patch that
removes reference_to_pointer_iterator from breakpoint_range:

      CXX    breakpoint.o
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c: In function ‘void download_tracepoint_locations()’:
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:11007:41: error: cannot allocate an object of abstract type ‘breakpoint’
    11007 |   for (breakpoint &b : all_tracepoints ())
          |                                         ^
    In file included from /home/smarchi/src/binutils-gdb/gdb/gdbthread.h:26,
                     from /home/smarchi/src/binutils-gdb/gdb/infrun.h:21,
                     from /home/smarchi/src/binutils-gdb/gdb/gdbarch.h:28,
                     from /home/smarchi/src/binutils-gdb/gdb/arch-utils.h:23,
                     from /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:21:
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.h:619:8: note:   because the following virtual functions are pure within ‘breakpoint’:
      619 | struct breakpoint : public intrusive_list_node<breakpoint>
          |        ^~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:250:1: note:     ‘virtual breakpoint::~breakpoint()’
      250 | breakpoint::~breakpoint ()
          | ^~~~~~~~~~

Change-Id: I05285ff27d21cb0ab80cba392ec4e959167e3cd7
Reviewed-By: Andrew Burgess <aburgess@redhat.com>