Add ambiguous reference incompatibility

[adpi2]

Found in https://github.com/47degrees/fetch

abstract class Document {
  def ::(hd: Document): Document  = DocCons(hd, this)
  
  def foo(bar: List[String]): String = bar match {
    case head :: tail => head
  }
}

[adpi2]

Actually this compatibility is a bug and is going to be fixed in the compiler itself: scala/scala3#11421 (comment)