RFR: 8265319: implement Sealed Classes as a standard feature in Java, javax.lang.model changes

[Vicente Romero]

Please review the changes needed in javax.lang.model API to make Sealed Classes a final feature in Java,

Thanks,

note: this PR is related to [PR-3526](https://github.com/openjdk/jdk/pull/3526)

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Commit messages:
 - 8265319: implement Sealed Classes as a standard feature in Java, javax.lang.model changes

Changes: https://git.openjdk.java.net/jdk/pull/3528/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3528&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265319
  Stats: 9 lines in 2 files changed: 0 ins; 6 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3528.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3528/head:pull/3528

PR: https://git.openjdk.java.net/jdk/pull/3528