RFR: 8265319: implement Sealed Classes as a standard feature in Java, javax.lang.model changes [v2]

[Vicente Romero]

> Please review the changes needed in javax.lang.model API to make Sealed Classes a final feature in Java. Please also review the related [CSR](https://bugs.openjdk.java.net/browse/JDK-8265320)
> 
> Thanks,
> 
> note: this PR is related to [PR-3526](https://github.com/openjdk/jdk/pull/3526)

Vicente Romero has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into JDK-8265319
 - 8265319: implement Sealed Classes as a standard feature in Java, javax.lang.model changes

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3528/files
  - new: https://git.openjdk.java.net/jdk/pull/3528/files/d814b556..543f485a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3528&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3528&range=00-01

  Stats: 563569 lines in 5123 files changed: 45774 ins; 510854 del; 6941 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