RFR: 8265130: Make ConstantDesc class hierarchy sealed [v3]

[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

Gavin Bierman has updated the pull request incrementally with one additional commit since the last revision:

  Removing file constants.patch added by mistake

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4135/files
  - new: https://git.openjdk.java.net/jdk/pull/4135/files/1cd54624..c8f632f6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4135&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4135&range=01-02

  Stats: 102 lines in 1 file changed: 0 ins; 102 del; 0 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