* We now never report derived-constraint type errors, even
  in the "insolubles".  See Note [Insoluble derived constraints]
  in TcRnTypes.

* The cec_suppress mechanism in TcErrors is refactored a bit so that:
   - We suppress *all* errors in unreachable code (they can be jolly
     confusing)
   - We no longer suppress *all* non-insoluble errors if there are *any
     insolubles anywhere.  Instead we are a bit more refined.
  See Note [Suppressing error messages] in TcErrors