RFR: 8351444: Shenandoah: Class Unloading may encounter recycled oops [v3]

[William Kemper]

> Unloading classes may require a walk of unreachable oops. For this reason, it is not safe to recycle memory before class unloading is complete. This complements existing code to prevent mutators from recycling trash regions while weak roots is in progress.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Trim extraneous comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23951/files
  - new: https://git.openjdk.org/jdk/pull/23951/files/a5db7360..1c73a85a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23951&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23951&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23951/head:pull/23951

PR: https://git.openjdk.org/jdk/pull/23951