[sealed-types] RFR: VM and core-libs sealed classes update

[Harold Seigel]

Push latest reviewed VM and core-libs changes into sealed-types repo.

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Commit messages:
 - VM and core-libs sealed classes update

Changes: https://git.openjdk.java.net/amber/pull/23/files
 Webrev: https://webrevs.openjdk.java.net/amber/23/webrev.00
  Stats: 140 lines in 14 files changed: 26 ins; 48 del; 66 mod
  Patch: https://git.openjdk.java.net/amber/pull/23.diff
  Fetch: git fetch https://git.openjdk.java.net/amber pull/23/head:pull/23

PR: https://git.openjdk.java.net/amber/pull/23