RFR: 8260517: implement Sealed Classes as a standard feature in Java [v7]

[Vicente Romero]

> Please review this PR that intents to make sealed classes a final feature in Java. This PR contains compiler and VM changes. In line with similar PRs, which has made preview features final, this one is mostly removing preview related comments from APIs plus options in test cases. Please also review the related [CSR](https://bugs.openjdk.java.net/browse/JDK-8265090)
> 
> Thanks
> 
> note: this PR is related to [PR-3528](https://github.com/openjdk/jdk/pull/3528) and must be integrated after it.

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 11 additional commits since the last revision:

 - Merge branch 'master' into JDK-8260517
 - restoring jdk.internal.javac.PreviewFeature.Feature.SEALED_CLASSES, it is needed by the build
 - Merge branch 'master' into JDK-8260517
 - Merge branch 'master' into JDK-8260517
 - updating comment after review feedback
 - removing javax.lang.model changes
 - Merge branch 'master' into JDK-8260517
 - Merge branch 'master' into JDK-8260517
 - fixing failing regression tests
 - JVM related changes
 - ... and 1 more: https://git.openjdk.java.net/jdk/compare/10336a26...0208dcf0

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3526/files
  - new: https://git.openjdk.java.net/jdk/pull/3526/files/304fd76a..0208dcf0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3526&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3526&range=05-06

  Stats: 18908 lines in 391 files changed: 9887 ins; 4814 del; 4207 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3526.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3526/head:pull/3526

PR: https://git.openjdk.java.net/jdk/pull/3526