[lworld] RFR: 8346680: [lworld] Field flattening when composing value classes should be revisited

[Frederic Parain]

Adjustments to flattening policy when composing atomic and non-atomic value types.

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Commit messages:
 - Fix volatile and abstract
 - Refine flattening policy for value composition

Changes: https://git.openjdk.org/valhalla/pull/1325/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1325&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346680
  Stats: 403 lines in 5 files changed: 386 ins; 7 del; 10 mod
  Patch: https://git.openjdk.org/valhalla/pull/1325.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1325/head:pull/1325

PR: https://git.openjdk.org/valhalla/pull/1325