RFR: 8330414: GenShen: Class unloading requires old regions be made parseable

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 6d09591099fa78a98a1f963987637138089f702e

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/40/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=40&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330414
  Stats: 339 lines in 19 files changed: 222 ins; 58 del; 59 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/40.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/40/head:pull/40

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/40