Trac #14000 showed up two errors

* In TcRnTypes.dropInsolubles we dropped all implications, which
  might contain the very insolubles we wanted to keep.  This was
  an outright error, and is why the out-of-scope error was actually
  lost altogether in Trac #14000

* In TcSimplify.simplifyInfer, if there are definite (insoluble)
  errors, it's better to suppress the following ambiguity test,
  because the type may be bogus anyway.  See TcSimplify
  Note [Quantification with errors].  This fix seems a bit clunky,
  but it'll do for now.