There were really two bugs
  a) When the fundep fires we must apply the matching
     substitution to the kinds of the remaining type vars
     (This happens in FunDeps.checkClsFD, when we create meta_tvs)

  b) When instantiating the un-matched type variables we must
     instantiate their kinds properly
     (This happens in TcSMonad.instFlexiTcS)

This fixes #6068 and #6015 (second reported bug).