largely based on the relevant portion of the unusedfields loplugin, but
adapted for local vars

Change-Id: Ic522a941573940e8f75c88f90ba5f37508ca49b1
Reviewed-on: https://gerrit.libreoffice.org/66835
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>