Compatibility with `-Xfuture` in type checking patmat

[liufengyun]

The following code fails with scalac -Xfuture test.scala, but Dotty compiles without problem:

object Test {
  case object Bob { override def equals(other: Any) = true }
  def f(x: Any): Bob.type = x match { case x @ Bob => x }
}

This is a separate issue extracted from #1463.