RFR: 8258239: Shenandoah: Used wrong closure to mark concurrent roots

[Zhengyu Gu]

During concurrent mark phase, there should not have forwarded objects. Therefore, it should use ShenandoahMarkRefsClosure to mark concurrent roots, instead of ShenandoahMarkResolveRefsClosure.

Note: this is *not* a correctness bug, but performance one, as ShenandoahMarkResolveRefsClosure closure unnecessarily resolves forwarding pointers, where always resolved to themselves.

- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8258239

Changes: https://git.openjdk.java.net/jdk/pull/1768/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1768&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258239
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1768.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1768/head:pull/1768

PR: https://git.openjdk.java.net/jdk/pull/1768