[lworld] RFR: 8329205: [lworld] Add jdk.internal.value.CheckedType API

[Mandy Chung]

This PR adds `jdk.internal.value.CheckedType` and other internal APIs for null-restricted value type and array support.   Also rename `Unsafe::isFlattenedArray` to `Unsafe::isFlatArray`.

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Commit messages:
 - 8329205: [lworld] Add jdk.internal.value.CheckedType API

Changes: https://git.openjdk.org/valhalla/pull/1065/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1065&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329205
  Stats: 853 lines in 32 files changed: 535 ins; 133 del; 185 mod
  Patch: https://git.openjdk.org/valhalla/pull/1065.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1065/head:pull/1065

PR: https://git.openjdk.org/valhalla/pull/1065