Inherit from tools::WeakBase non-virtually, so that we can use a
static_cast in tools::WeakReference::get instead of a dynamic_cast.

This takes the file-open time from 1m21 to 40s for me.

Add a clang plugin to make sure we don't accidentally end up inheriting
from tools::WeakBase more than once.

Change-Id: I9c7c36403333f99094e1f9d8cce2ecd9200377f9
Reviewed-on: https://gerrit.libreoffice.org/71231Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>