Add changes to ensure that a DebugInfo that has been archived cannot be archived...

Add changes to ensure that a DebugInfo that has been archived cannot be archived again.

* discard_or_archive_marked_DebugInfos: clear the mark bit for a Debuginfo
  that will be archived

* discard_DebugInfos_which_overlap_with: when selecting DebugInfos to be
  discarded or archived, fix a mistake in which some mark bits wouldn't be
  changed at all, meaning their "old" value was used to influence the current
  operation.

These may (or may not) fix #393146; at the very least, they are somehow
related.