RFR: add the NullRestricted annotation to null restricted fields of value classes

[Vicente Romero]

null restricted fields of value classes should be annotated, temporarily, with the @NullRestricted annotation

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Commit messages:
 - add the NullRestricted annotation to null restricted fields of value classes

Changes: https://git.openjdk.org/valhalla/pull/1918/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1918&range=00
  Stats: 68 lines in 8 files changed: 22 ins; 15 del; 31 mod
  Patch: https://git.openjdk.org/valhalla/pull/1918.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1918/head:pull/1918

PR: https://git.openjdk.org/valhalla/pull/1918