Use a LazyKeySet for LinkedHashMap's keySet

LinkedHashMap's `keySet` has always been returning a LinkedKeySet, a private data structure that was extended from the
generic MapOps.KeySet. Recently MapOps.KeySet was changed to a strict data structure, so we change the `keySet` method to
use the lazy wrapper instead. Unfortunately we cannot change the hierarchy of `LinkedKeySet`, but the API never said we have to use it.

Author: natsukagami

library/src/scala/collection/Map.scala

@@ -426,7 +426,7 @@ object MapOps {
 
   /** The implementation class of the set returned by `keySet`, for pure maps.
     */
-  private class LazyKeySet[K, +V, +CC[_, _] <: IterableOps[?, AnyConstr, ?], +C](mp: MapOps[K, V, CC, C]) extends AbstractSet[K] with DefaultSerializable {
+  private[collection] class LazyKeySet[K, +V, +CC[_, _] <: IterableOps[?, AnyConstr, ?], +C](mp: MapOps[K, V, CC, C]) extends AbstractSet[K] with DefaultSerializable {
     def iterator: Iterator[K] = mp.keysIterator
     def diff(that: Set[K]): Set[K] = LazyKeySet.this.fromSpecific(this.view.filterNot(that))
     def contains(key: K): Boolean = mp.contains(key)

library/src/scala/collection/mutable/LinkedHashMap.scala

@@ -234,11 +234,14 @@ class LinkedHashMap[K, V]
       def extract(nd: Entry): (K, V) = (nd.key, nd.value)
     }
 
+  /** Note that a LinkedKeySet could be strict. */
   protected class LinkedKeySet extends KeySet {
     override def iterableFactory: IterableFactory[collection.Set] = LinkedHashSet
   }
 
-  override def keySet: collection.Set[K] = new LinkedKeySet
+  override def keySet: collection.Set[K] = new MapOps.LazyKeySet(this) {
+    override def iterableFactory: IterableFactory[collection.Set] = LinkedHashSet
+  }
 
   override def keysIterator: Iterator[K] =
     if (size == 0) Iterator.empty