[lworld] RFR: 8378476: [lworld] Disable Character cache for --enable-preview

[Roger Riggs]

Disable Character.valueOf cache for --enable-preview; always invokes `new Character(c)`.
Refactor Boolean.valueOf using same code structure as other disables of caches.
This aligns with Boolean, Byte, Short, Integer, Long disabling of wrapper classes with --enable-preview.

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Commit messages:
 - 8378476i: [lworld] Disable Character cache for --enable-preview

Changes: https://git.openjdk.org/valhalla/pull/2167/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2167&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378476
  Stats: 11 lines in 2 files changed: 5 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/valhalla/pull/2167.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2167/head:pull/2167

PR: https://git.openjdk.org/valhalla/pull/2167