dwarf-refcmp

[Roland McGrath <roland at redhat dot com>]

I stashed another unfinished tidbit on roland/dwarf-refcmp (not intended
for a real commit).  This is the start of the approach for reference
tracking.  

The beginning piece that's started there is to use a different
control/dispatch flow for the whole-file comparison.  Instead of methods
(operator==) on debug_info_entry et al, the recursion is done via methods
on the dwarf_comparator object.  This gives us a place to hang the ref
tracking data structures outside the dwarf object itself, where the
deep-in-recursion calls can refer to them and the tree-walking steps can
populate them.  This sort of structure for comparison is probably also a
better way to implement dwarfcmp.cc's describe_mismatch.

I think this is the way to work out the ref tracker first.  Then we'll use
it in dwarf_edit/dwarf_output construction, with a similar top-down control
flow.


Thanks,
Roland