Disambiguate record selectors by type signature
This makes DuplicateRecordFields more liberal in when it will
accept ambiguous record selectors, making use of type information in a
similar way to updates. See Note [Disambiguating record fields] for more
details. I've also refactored how record updates are disambiguated.
Test Plan: New and amended tests in overloadedrecflds
Reviewers: simonpj, goldfire, bgamari, austin
Differential Revision: https://phabricator.haskell.org/D1391