RFR: 8279540: Shenandoah: Should only clear CLD::_claim_strong mark for strong CLD iterations

[Zhengyu Gu]

JDK-8273559 started to use CLD::_claim_other for heap iteration in supporting multi-threaded heap walk. For regular CLD, it still uses CLD::_claim_strong, but for those walks, it clears all claim bits, instead of just _claim_strong bit.

I don't think it is a fatal bug, it just means heap iteration may walk a CLD by multiple workers, which impacts performance.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - 8279540: Shenandoah: Should only clear CLD::_claim_strong mark for strong CLD iterations

Changes: https://git.openjdk.java.net/jdk/pull/6980/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6980&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279540
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6980.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6980/head:pull/6980

PR: https://git.openjdk.java.net/jdk/pull/6980