8265130: Make ConstantDesc class hierarchy sealed

Reviewed-by: mchung, jvernee, vromero

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

@@ -49,18 +49,15 @@ import static java.util.stream.Collectors.joining;
  * {@linkplain ClassDesc} for the component type and then call the {@link #arrayType()}
  * or {@link #arrayType(int)} methods.
  *
- * @apiNote In the future, if the Java language permits, {@linkplain ClassDesc}
- * may become a {@code sealed} interface, which would prohibit subclassing except
- * by explicitly permitted types.  Non-platform classes should not implement
- * {@linkplain ClassDesc} directly.
- *
  * @see ConstantDescs
  *
  * @since 12
  */
-public interface ClassDesc
+public sealed interface ClassDesc
         extends ConstantDesc,
-                TypeDescriptor.OfField<ClassDesc> {
+                TypeDescriptor.OfField<ClassDesc>
+        permits PrimitiveClassDescImpl,
+                ReferenceClassDescImpl {
 
     /**
      * Returns a {@linkplain ClassDesc} for a class or interface type,