Currently, one can document top-level instance declarations, but derived
instances (both those in `deriving` clauses and standalone `deriving`
instances) do not enjoy the same privilege. This makes the necessary
changes to the parser to enable attaching Haddock comments for derived
instances.

Updates haddock submodule.

Fixes #11768.

Test Plan: ./validate

Reviewers: hvr, bgamari, austin

Reviewed By: austin

Subscribers: thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2175

GHC Trac Issues: #11768