RFR: 8265130: Make ConstantDesc class hierarchy sealed

[Gavin Bierman]

Hi all,

Please review this patch to make the ConstantDesc hierarchy `sealed`, as was promised in its Javadoc, now that sealed classes are finalising in JDK 17. 

Thanks,
Gavin

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Commit messages:
 - 8265130: Make ConstantDesc class hierarchy sealed

Changes: https://git.openjdk.java.net/jdk/pull/4135/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4135&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265130
  Stats: 25 lines in 6 files changed: 16 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4135.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4135/head:pull/4135

PR: https://git.openjdk.java.net/jdk/pull/4135