...to flag places that implicitly derive a class privately instead of publicly,
where accidental private derivation can cause unexpected failure of
dynamic_cast, cf. 63b67ab5cab8cf7576a68cabe5fb1a42c6ad800c "Use public
derivation, and remove then-unnecessary downcasts."
Change-Id: I4bcd5c79c7e27380c820e2dd072fa4c4e8980078