Forward-port of scala/scala#11065

In `<a><b/><c/><a>`, a `NodeBuffer` (which extends `ArrayBuffer`) is
used to accumulate the children. The buffer is passed to `new Elem($buf:
_*)`, which only works thanks to the implicit `collection.Seq[Node] =>
NodeSeq` declared in scala-xml.

With `-Vprint:typer`:

```scala
scala> <a><b/></a>
[[syntax trees at end of                     typer]] // rs$line$1

    val res0: scala.xml.Elem =
      {
        new _root_.scala.xml.Elem(null, "a", _root_.scala.xml.Null,
          scala.xml.TopScope, false,
          {
            val $buf: scala.xml.NodeBuffer = new _root_.scala.xml.NodeBuffer()
            $buf.&+(
              {
                {
                  new _root_.scala.xml.Elem(null, "b", _root_.scala.xml.Null,
                    scala.xml.TopScope, true, [ : scala.xml.Node]*)
                }
              }
            )
            scala.xml.NodeSeq.seqToNodeSeq($buf)
          }*
        )
      }
```

The implicit was not inserted in 2.12 because the varargs parameter of
Elem accepted a `collection.Seq`.