Flatten nested capture sets in retainedElementsRaw

[odersky]

Without the added clause, i23570 looks like this after typer:

[[syntax trees at end of                     typer]] // i23570.scala
package <empty> {
  final lazy module val i23570$package: i23570$package = new i23570$package()
  final module class i23570$package() extends Object() {
    this: i23570$package.type =>
    def f[C >: scala.caps.CapSet <: scala.caps.CapSet^{cap}](
      xs: List[(() -> Unit)^{C}]): List[(() -> Unit)^{C}] = xs.reverse
    def test(io: Object^{cap}, async: Object^{cap}): Unit =
      {
        val ok: List[() ->{scala.caps.CapSet^{io}} Unit] =
          f[_root_.scala.caps.CapSet^{io}](Nil)
        val x:
          List[() ->{scala.caps.CapSet^{io}} Unit] ->
            List[() ->{scala.caps.CapSet^{io}} Unit]
         = (xs: List[() ->{scala.caps.CapSet^{io}} Unit]) =>
          f[_root_.scala.caps.CapSet^{io}](xs)
        val y:
          List[() ->{scala.caps.CapSet^{io, async}} Unit] ->
            List[() ->{scala.caps.CapSet^{io, async}} Unit]
         = (xs: List[() ->{scala.caps.CapSet^{io, async}} Unit]) =>
          f[_root_.scala.caps.CapSet^{io, async}](xs)
        ()
      }
  }
}

Note the nested capture sets in the types of x and y.

Fixes #23570

[noti0na1]

Do we need isRetainsLike here?

[odersky]

Ni, I don't think a call-by-name set can end up here.

[noti0na1]

LGTM