8265130: Make ConstantDesc class hierarchy sealed

Reviewed-by: mchung, jvernee, vromero

src/java.base/share/classes/java/lang/constant/MethodHandleDesc.java

@@ -35,15 +35,12 @@ import static java.lang.constant.DirectMethodHandleDesc.Kind.CONSTRUCTOR;
  * A <a href="package-summary.html#nominal">nominal descriptor</a> for a
  * {@link MethodHandle} constant.
  *
- * @apiNote In the future, if the Java language permits, {@linkplain MethodHandleDesc}
- * may become a {@code sealed} interface, which would prohibit subclassing except
- * by explicitly permitted types.  Non-platform classes should not implement
- * {@linkplain MethodHandleDesc} directly.
- *
  * @since 12
  */
-public interface MethodHandleDesc
-        extends ConstantDesc {
+public sealed interface MethodHandleDesc
+        extends ConstantDesc
+        permits AsTypeMethodHandleDesc,
+                DirectMethodHandleDesc {
 
     /**
      * Creates a {@linkplain MethodHandleDesc} corresponding to an invocation of a