Warn about sequences being passed to repeated parameters without :_*

Ha, looks like we're not immune from these problems:

```
scala> class C { List(null : _*) }
defined class C

scala> class C { List(??? : _*) }
java.lang.AssertionError: assertion failed:
  transformCaseApply: name = ??? tree = $qmark$qmark$qmark / class scala.reflect.internal.Trees$Ident
```