found a couple more by running
    make build-nocheck
to exclude testing code

Also, teach unusedfields loplugin about operator<<, since
referring to a field in an ostream operator<< does not indicate a
real use, it's normally for debugging

Change-Id: I3dce22bf5afda2fd09e01de9bf1d0ef85e535aa8
Reviewed-on: https://gerrit.libreoffice.org/39625Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>