Wrong class tag for ClassTag[Unit]

```scala
object Foo {
  def main(args: Array[String]): Unit = {
    println(implicitly[reflect.ClassTag[Unit]])
    println(implicitly[reflect.ClassTag[Unit]].getClass)
  }
}
```

on Dotty prints

```
scala.runtime.BoxedUnit
class scala.reflect.ClassTag$GenericClassTag
```
and on scalac prints
```
Unit
class scala.reflect.ManifestFactory$$anon$14
```