Make ConstantDesc class hierarchy sealed

The class hierarchy rooted at `ConstantDesc` in package `java.language.constant` is used to denote a nominal descriptor for a loadable constant value, as defined in JVMS 4.4.

The javadoc for these classes mentions that this hierarchy may become sealed if supported by the Java language, for example:

* @apiNote In the future, if the Java language permits, {@linkplain ConstantDesc}
 * may become a {@code sealed} interface, which would prohibit subclassing except by
 * explicitly permitted types. Clients can assume that the following
 * set of subtypes is exhaustive: {@link String}, {@link Integer},
 * {@link Long}, {@link Float}, {@link Double}, {@link ClassDesc},
 * {@link MethodTypeDesc}, {@link MethodHandleDesc}, and
 * {@link DynamicConstantDesc}; this list may be extended to reflect future
 * changes to the constant pool format as defined in JVMS 4.4.
 

Now JEP 409 proposes that sealed classes become a final feature, we can amend the class hierarchy to be sealed.