Flatten nested capture sets in retainedElementsRaw (#23571)

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

```scala
[[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

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -94,6 +94,8 @@ extension (tp: Type)
   def retainedElementsRaw(using Context): List[Type] = tp match
     case OrType(tp1, tp2) =>
       tp1.retainedElementsRaw ++ tp2.retainedElementsRaw
+    case AnnotatedType(tp1, ann) if tp1.derivesFrom(defn.Caps_CapSet) && ann.symbol.isRetains =>
+      ann.tree.retainedSet.retainedElementsRaw
     case tp =>
       // Nothing is a special type to represent the empty set
       if tp.isNothingType then Nil

tests/pos-custom-args/captures/i23570.scala

@@ -0,0 +1,7 @@
+def f[C^](xs: List[() ->{C} Unit]): List[() ->{C} Unit] =
+  xs.reverse
+
+def test(io: Object^, async: Object^): Unit =
+  val ok = f[{io}](Nil)
+  val x = f[{io}]        // was error
+  val y = f[{io, async}] // was error

tests/pos/cc-use-alternatives.scala

@@ -0,0 +1,24 @@
+import language.experimental.captureChecking
+// no separation checking
+import caps.{cap, use}
+
+def foo1(@use xs: List[() => Unit]): Unit =
+  var x: () ->{xs*} Unit = xs.head
+  var ys = xs
+  while ys.nonEmpty do
+    ys = ys.tail
+    x = ys.head
+
+def foo2(@use xs: List[() => Unit]): Unit =
+  var x: () => Unit = xs.head // note: this would fail separation checking
+  var ys = xs
+  while ys.nonEmpty do
+    ys = ys.tail
+    x = ys.head
+
+def foo3[@use C^](xs: List[() ->{C} Unit]): Unit =
+  var x: () ->{C} Unit = xs.head
+  var ys = xs
+  while ys.nonEmpty do
+    ys = ys.tail
+    x = ys.head