RFR: 8373203: Genshen: Non-strong reference leak in old gen

[William Kemper]

The generational mode for Shenandoah will collect _referents_ for the generation being collected. For example, if we have a young reference pointing to an old referent, that young reference will be processed after we finish marking the old generation. This presents a problem for discovery.
  
When the young mark _encounters_ a young reference with an old referent, it cannot _discover_ it because old marking hasn't finished. However, if it does not discover it, the old referent will be strongly marked. This, in turn, will prevent the old generation from clearing the referent (if it even reaches it again during old marking).
  
To solve this, we let young reference processing discover the old reference by having it use the old generation reference processor to do so. This means the old reference processor can have a discovered list that contains young weak references. If any of these young references reside in a region that is collected, old reference processing will crash when it processes such a reference. Therefore, we add a method `heal_discovered_lists` to traverse the discovered lists after young evacuation is complete. The method will replace any forwarded entries in the discovered list with the forwardee.

This PR also extends whitebox testing support for Shenandoah, giving us the ability to trigger young/old collections and interrogate some properties of heaps and regions.

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Commit messages:
 - Oops, change name of class in test xdoc
 - Oops, initialize new field
 - Add missing newline
 - Add comments, fix typo, survive multiple young GCs
 - WIP: checkpoint
 - Clean up test, increase likelihood of cross generational references
 - Add test for genshen reference processing

Changes: https://git.openjdk.org/jdk/pull/28810/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28810&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373203
  Stats: 472 lines in 14 files changed: 437 ins; 18 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/28810.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28810/head:pull/28810

PR: https://git.openjdk.org/jdk/pull/28810