RFR: 8373203: Genshen: Non-strong reference leak in old gen [v7]

[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.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 22 commits:

 - Merge remote-tracking branch 'jdk/master' into fix-old-reference-processing
 - Heal discovered lists for any young collection coincides with old marking
 - Configure thread local mark closure on delegated old reference processor
 - Merge remote-tracking branch 'jdk/master' into fix-old-reference-processing
 - Fix idiosyncratic white space in whitebox
   
   Co-authored-by: Stefan Karlsson <stefan.karlsson at oracle.com>
 - Sort includes
 - Heal old discovered lists in parallel
 - Fix comment
 - Factor duplicate code into shared method
 - Heal discovered oops in common place for degen and concurrent update refs
 - ... and 12 more: https://git.openjdk.org/jdk/compare/4458cab4...ed0d0272

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Changes: https://git.openjdk.org/jdk/pull/28810/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28810&range=06
  Stats: 669 lines in 20 files changed: 537 ins; 84 del; 48 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