RFR: Implement ShenandoahHeap::requires_barriers()

[Roman Kennke]

This add proper implementation of requires_barriers() for ShenandoahHeap, and adds some comments to CollectedHeap about it.

Testing (with a few other fixes in the pipeline):
 - [x] java/lang/Continuation (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)
 - [x] java/lang/Thread/virtual  (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)

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Commit messages:
 - Implement ShenandoahHeap::requires_barriers()

Changes: https://git.openjdk.java.net/loom/pull/29/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=29&range=00
  Stats: 16 lines in 3 files changed: 13 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/loom/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/29/head:pull/29

PR: https://git.openjdk.java.net/loom/pull/29