collection of heuristics to look for local variables that are never read
from i.e. do not contribute to the surrounding logic

This is an expensive plugin, since it walks up the parent tree,
so it is off by default.

Change-Id: Ib8ba292241bd16adf299e8bba4502cb473513a06
Reviewed-on: https://gerrit.libreoffice.org/52450Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>