[lworld] RFR: 8374800: [lworld] Add a NULLABLE_NON_ATOMIC_FLAT layout

[Frederic Parain]

Strict final instance fields are not subject to concurrent writes during a read access, so they can be flattened even if they are nullable and bigger than 64 bits. The NULLABLE_NON_ATOMIC_FLAT layout is added for this particular case.
This new layout can also be used in the special case of nullable empty value classes, because their payload contains a single entry, the null-marker, which is naturally atomic.

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

Commit messages:
 - Cleanups and fixes
 - Fix Copyright years
 - Fix compilation
 - Merge remote-tracking branch 'upstream/lworld' into flat_final
 - Improve tests
 - Merge remote-tracking branch 'upstream/lworld' into flat_final
 - Merge remote-tracking branch 'upstream/lworld' into flat_final
 - Add empty values case and some LayoutKindHelper cleanup
 - Merge remote-tracking branch 'upstream/lworld' into flat_final
 - Fix build after merge
 - ... and 3 more: https://git.openjdk.org/valhalla/compare/093d033c...972768c8

Changes: https://git.openjdk.org/valhalla/pull/1407/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1407&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374800
  Stats: 673 lines in 16 files changed: 589 ins; 10 del; 74 mod
  Patch: https://git.openjdk.org/valhalla/pull/1407.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1407/head:pull/1407

PR: https://git.openjdk.org/valhalla/pull/1407