RFR: 8328611: Thread safety issue in com.sun.tools.jdi.ReferenceTypeImpl::classObject

[Chris Plummer]

Fixed "double-checked-locking" bug in `ReferenceImplType.classObject()`. Details in the first comment. Tested with the following:
- com/sun/jdi
- nsk/jdi
- nsk/jdwp
- nsk/jdb

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Commit messages:
 - Update copyright
 - Unified 'worst case' comment.
 - Fix volatile issue with classObject() API.

Changes: https://git.openjdk.org/jdk/pull/19439/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19439&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328611
  Stats: 26 lines in 1 file changed: 2 ins; 13 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/19439.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19439/head:pull/19439

PR: https://git.openjdk.org/jdk/pull/19439