[lworld] RFR: 8346303: [lworld] Layout builder should not reuse gap before inherited fields

[Frederic Parain]

Small fix to prevent a layout decision that can negatively impact the support of atomic layouts.

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Commit messages:
 - Don't try to reuse gap between header and inherited fields

Changes: https://git.openjdk.org/valhalla/pull/1319/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1319&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346303
  Stats: 13 lines in 1 file changed: 0 ins; 12 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1319.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1319/head:pull/1319

PR: https://git.openjdk.org/valhalla/pull/1319