Array[_ <: Foo] where Foo is a universal trait should be erased to Object

Currently, it's erased to `Array[Foo]`:

``` scala
trait Foo extends Any

object Univ {
  def univ[T <: Foo](x: Array[T]) = {}
  def univ2(x: Array[_ <: Foo]) = {}
}
```

In `scalac` it was erased to Object, I don't know why. In Dotty we need to erase it to Object if we want a different representation for arrays of value classes
