Capture stripping for capture variables

Given a capture set variable `C`, its `stripCapturing` should be just `CapSet`.

Author: Linyxus

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

@@ -359,6 +359,8 @@ extension (tp: Type)
       parent.stripCapturing
     case atd @ AnnotatedType(parent, annot) =>
       atd.derivedAnnotatedType(parent.stripCapturing, annot)
+    case tp if tp.derivesFrom(defn.Caps_CapSet) =>
+      defn.Caps_CapSet.typeRef
     case _ =>
       tp