RFR: 8371083: FollowReferences reports non-class objects as JVMTI_HEAP_REFERENCE_SYSTEM_CLASS

[Alex Menkov]

FollowReferences with null initial_object starts heap walking from "heap roots", which include system classes.
All oops from ClassLoaderDataGraph are reported with JVMTI_HEAP_REFERENCE_SYSTEM_CLASS kind, but some of the objects are not classes.
The fix updates FollowReferences to report non-class objects from ClassLoaderDataGraph as JVMTI_HEAP_REFERENCE_OTHER

Testing: tier1..4,hs-tier5-svc

-------------

Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/28224/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28224&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371083
  Stats: 211 lines in 3 files changed: 208 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28224.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28224/head:pull/28224

PR: https://git.openjdk.org/jdk/pull/28224