[jdk25u] RFR: 8364111: InstanceMirrorKlass iterators should handle CDS and hidden classes consistently

[Aleksey Shipilev]

Improves GC reliability with CDS and hidden classes, unblocks Shenandoah improvements that use these iterators.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `all`

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Commit messages:
 - Backport ebb7f5d39be8497fc89e25d0905335102e12c063

Changes: https://git.openjdk.org/jdk25u/pull/205/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=205&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364111
  Stats: 64 lines in 2 files changed: 32 ins; 30 del; 2 mod
  Patch: https://git.openjdk.org/jdk25u/pull/205.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/205/head:pull/205

PR: https://git.openjdk.org/jdk25u/pull/205