gdbsupport: add missing increment/decrement operators to reference_to_pointer_iterator

gdbsupport: add missing increment/decrement operators to reference_to_pointer_iterator

Using the following patch, I would get this build failure:

      CXX    breakpoint.o
    In file included from /usr/include/c++/13.1.1/bits/stl_algobase.h:66,
                     from /usr/include/c++/13.1.1/bits/hashtable_policy.h:36,
                     from /usr/include/c++/13.1.1/bits/hashtable.h:35,
                     from /usr/include/c++/13.1.1/bits/unordered_map.h:33,
                     from /usr/include/c++/13.1.1/unordered_map:41,
                     from /usr/include/c++/13.1.1/functional:63,
                     from /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/ptid.h:35,
                     from /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/common-defs.h:206,
                     from /home/smarchi/src/binutils-gdb/gdb/defs.h:26,
                     from /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:20:
    /usr/include/c++/13.1.1/bits/stl_iterator_base_funcs.h: In instantiation of ‘constexpr void std::__advance(_BidirectionalIterator&, _Distance, bidirectional_iterator_tag) [with _BidirectionalIterator = reference_to_pointer_iterator<intrusive_list_iterator<bp_location, intrusive_base_node<bp_location> > >; _Distance = long int]’:
    /usr/include/c++/13.1.1/bits/stl_iterator_base_funcs.h:224:21:   required from ‘constexpr void std::advance(_InputIterator&, _Distance) [with _InputIterator = reference_to_pointer_iterator<intrusive_list_iterator<bp_location, intrusive_base_node<bp_location> > >; _Distance = long int]’
    /usr/include/c++/13.1.1/bits/stl_iterator_base_funcs.h:237:19:   required from ‘constexpr _InputIterator std::next(_InputIterator, typename iterator_traits<_Iter>::difference_type) [with _InputIterator = reference_to_pointer_iterator<intrusive_list_iterator<bp_location, intrusive_base_node<bp_location> > >; typename iterator_traits<_Iter>::difference_type = long int]’
    /home/smarchi/src/binutils-gdb/gdb/breakpoint.c:1073:19:   required from here
    /usr/include/c++/13.1.1/bits/stl_iterator_base_funcs.h:179:11: error: no match for ‘operator--’ (operand type is ‘reference_to_pointer_iterator<intrusive_list_iterator<bp_location, intrusive_base_node<bp_location> > >’)
      179 |           --__i;
          |           ^~~~~

This points out that while intrusive_list_iterator has an operator--,
the reference_to_pointer_iterator wrapper does not.  I'm not to sure why
the compiler chooses the overload of __advance that accepts a
_BidirectionalIterator, given that reference_to_pointer_iterator can't
be decremented, but adding those operators seems like the right thing to
do in any case, for completeness.

Change-Id: I8e2044b6734fadf0f21093047cf35bb7080dbdc3
Reviewed-By: Andrew Burgess <aburgess@redhat.com>