[libc++] Consistency fixes for the CSV status files (#101572)

- Make sure we always have the same number of entries. In particular, I
  dropped the "Group" entry that was tracked for papers but not for LWG
  issues, since I don't think that added much. The group is already
  tracked on the paper itself.
- Add "Label" row to all the CSV files.

This is mechanical, but it's necessary if we want to process our CSV
files uniformly, for example to synchronize their state with Github
issues.

Author: ldionne