Fix two CC errors in stdlib

They turns out not to be bugs. We cannot safely widen `{asIterable*}` to
`{}` in these cases.

Author: Linyxus

scala2-library-cc/src/scala/collection/Iterable.scala

@@ -684,7 +684,7 @@ trait IterableOps[+A, +CC[_], +C] extends Any with IterableOnce[A] with Iterable
 
   def flatMap[B](@caps.use f: A => IterableOnce[B]^): CC[B]^{this, f*} = iterableFactory.from(new View.FlatMap(this, f))
 
-  def flatten[B](implicit asIterable: A -> IterableOnce[B]): CC[B]^{this} = flatMap(asIterable)
+  def flatten[B](implicit asIterable: A -> IterableOnce[B]): CC[B]^{this, asIterable*} = flatMap(asIterable)
 
   def collect[B](pf: PartialFunction[A, B]^): CC[B]^{this, pf} =
     iterableFactory.from(new View.Collect(this, pf))

scala2-library-cc/src/scala/collection/immutable/LazyListIterable.scala

@@ -600,7 +600,7 @@ final class LazyListIterable[+A] private(@untrackedCaptures lazyState: () => Laz
     *
     * $preservesLaziness
     */
-  override def flatten[B](implicit asIterable: A -> IterableOnce[B]): LazyListIterable[B]^{this} = flatMap(asIterable)
+  override def flatten[B](implicit asIterable: A -> IterableOnce[B]): LazyListIterable[B]^{this, asIterable*} = flatMap(asIterable)
 
   /** @inheritdoc
     *