[jdk17u-dev] RFR: 8306929: Avoid CleanClassLoaderDataMetaspaces safepoints when previous versions are shared

[Oli Gillespie]

Hi all,

This pull request contains a backport of commit [408cec51](https://github.com/openjdk/jdk/commit/408cec516bb5fd82fb6dcddeee934ac0c5ecffaf) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by Stefan Johansson on 4 May 2023 and was reviewed by Coleen Phillimore and Serguei Spitsyn.

It's clean except for the surrounding context changing between NULL and nullptr which was trivial to resolve. It comes with its own test, but I also confirmed that it fixes the reproducer shared on the issue.

Thanks!

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Commit messages:
 - Backport 408cec516bb5fd82fb6dcddeee934ac0c5ecffaf

Changes: https://git.openjdk.org/jdk17u-dev/pull/2776/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2776&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306929
  Stats: 150 lines in 6 files changed: 127 ins; 3 del; 20 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2776.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2776/head:pull/2776

PR: https://git.openjdk.org/jdk17u-dev/pull/2776