Advanced ID copying code can now take a `new_owner_id` ID pointer parameter,
and use it to set the relevant 'loopback' pointer to its owner ID by the
copy code itself.
Besides avoiding the need for all code copying embedded IDs to set the
loopback pointer themselves, this also means that `lib_id` copying code
itself does not need to use `IDWALK_IGNORE_MISSING_OWNER_ID` anymore.
This change is not expected to have any effect in current codebase.
ec350a6115