[lworld] RFR: JDK-8244315 [lworld] The hierarchy between V$ref and V$val must be sealed

[Jim Laskey]

- Remove final flag from V$ref.
- Set V$ref as SEALED.
- Add value V$val to list of V$ref's permitted classes.

Test ensures that the permitted classes of Point$ref is exactly Point$val ("Point").

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Commit messages:
 - JDK-8244315 [lworld] The hierarchy between V$ref and V$val must be sealed.

Changes: https://git.openjdk.java.net/valhalla/pull/123/files
 Webrev: https://webrevs.openjdk.java.net/valhalla/123/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244315
  Stats: 51 lines in 2 files changed: 50 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/123.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/123/head:pull/123

PR: https://git.openjdk.java.net/valhalla/pull/123