Spurious "private default argument... is never used" warning with -Xlint

```
object O { O() }
case class O private (x: Int = 3)
```

gives (in 2.11.0-M4)

```
warning: private default argument in object O is never used
       case class O private (x: Int = 3)
                             ^
```

but it is; it's used from the companion object.