RFR: 8256011: Shenandoah: Don't resurrect finalizably reachable objects

[Roman Kennke]

In the weak-LRB we currently return referents when it is 'marked', that is when it's either reachable strongly or through a finalizable object. This means a finalizable object can be resurrected by Reference.get(), which is wrong. Only truly strongly reachable objects should be returned by Reference.get() during weak-reference-processing. 

Testing: hotspot_gc_shenandoah, tier1 & tier2 with -XX:+UseShenandoahGC

-------------

Commit messages:
 - 8256011: Shenandoah: Don't resurrect finalizably reachable objects

Changes: https://git.openjdk.java.net/jdk/pull/1109/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1109&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256011
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1109.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1109/head:pull/1109

PR: https://git.openjdk.java.net/jdk/pull/1109