RFR: 8315044: GenShen: Verifier detects clean card should be dirty [v5]

[Kelvin Nilsen]

> When a Reference object is newly discovered, it is placed onto the worker's thread-local discovered list.  This sometimes results in a reference from an old object to a young object, requiring that the remembered set card-table entry be marked as dirty.  This patch causes the marking to be performed.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Use card_mark_barrier() method and ShenandoahCardBarrier tests

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/314/files
  - new: https://git.openjdk.org/shenandoah/pull/314/files/f375d571..2618ff3c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=314&range=04
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=314&range=03-04

  Stats: 11 lines in 1 file changed: 5 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah/pull/314.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/314/head:pull/314

PR: https://git.openjdk.org/shenandoah/pull/314