Match exhaustion warning on literal type

```scala
class C {
  def m(x: true) = x match {
    case true => println("the one true path")
  }
}
```

```
14:53:27 $ scala exhaustive-literals.scala
exhaustive-literals.scala:2: warning: match may not be exhaustive.
It would fail on the following input: false
  def m(x: true) = x match {
                   ^
```

Scala 2.13.0