RFR: 8343227: GenShen: Fold resource mark into management of preselected regions

[William Kemper]

`ShenandoahCollectionSetPreselector` manages a resource scoped array. It makes sense to move the `ResourceMark` into this class.

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Commit messages:
 - Move resource mark into collection set preselector

Changes: https://git.openjdk.org/shenandoah/pull/530/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=530&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343227
  Stats: 3 lines in 2 files changed: 2 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/530.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/530/head:pull/530

PR: https://git.openjdk.org/shenandoah/pull/530