[lworld] RFR: 8326990: [lworld] make value classes a preview feature

[Vicente Romero]

Javac changes to make value classes a preview feature,

TIA

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Commit messages:
 - modifying regression tests
 - minor changes
 - Merge lworld
 - 8326990: [lworld] make value classes a preview feature

Changes: https://git.openjdk.org/valhalla/pull/1037/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1037&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326990
  Stats: 118 lines in 31 files changed: 51 ins; 43 del; 24 mod
  Patch: https://git.openjdk.org/valhalla/pull/1037.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1037/head:pull/1037

PR: https://git.openjdk.org/valhalla/pull/1037