[jdk21u-dev] RFR: 8338939: Simplify processing of hidden class names

[Aleksey Shipilev]

Makes JFR more durable by avoiding Java mirror accesses for the classes that might be unloading. We were seeing the problems (crashes) at least with Shenandoah in both mainline and JDK 21u. There seem to be no bugtail for this change since JDK 24 integration 11 months ago.

Additional testing:
 - [x] Linux AArch64 server fastdebug, `jdk/jfr`
 - [x] Linux AArch64 server fastdebug, `jdk/jfr` + `-XX:+UseShenandoahGC`

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Commit messages:
 - Backport 0f667103db7842fe9d3399f56baee0a5def4529e

Changes: https://git.openjdk.org/jdk21u-dev/pull/2018/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338939
  Stats: 59 lines in 2 files changed: 2 ins; 40 del; 17 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2018.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2018/head:pull/2018

PR: https://git.openjdk.org/jdk21u-dev/pull/2018