Better errors on duplicate record fields

[cocreature]

Currently, duplicate record fields only blow up in the daml-lf typechecker or indirectly when they are used somewhere. We should really produce a better error more directly. Unfortunately, GHC itself accepts the following program:

{-# LANGUAGE DuplicateRecordFields, DisambiguateRecordFields #-}
module Test where

data X = X { a :: Int, a :: Int }

It seems reasonable to add this check to our preprocessor.