A user_id_t carries the same information as a DIERef, but it takes up
Furthermore, DIERef::operator<'s implementation is very
questionable, as it does not take the cu_offset and section fields into
account. Using just the die offset was correct it the days when all
debug info lived in a single section, but since we started supporting
DWO debug info, this was no longer true. The comparison operator could
be fixed, but it seems like using the user_id_t for these purposes is a
better idea overall.
I think this did not cause any bugs, because the only place the
comparison operator was used is in m_function_scope_qualified_name_map,
and this one is local to a dwo file, but I am not 100% sure of that.