RFR: 8344779: GenShen: Consolidate shared heap region closures into one header

[William Kemper]

Move shared heap region closure implementations into one header, move non-shared implementations closer to their only usage.

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Commit messages:
 - Consolidate shared heap region closures into one header

Changes: https://git.openjdk.org/shenandoah/pull/549/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=549&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344779
  Stats: 351 lines in 9 files changed: 152 ins; 195 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah/pull/549.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/549/head:pull/549

PR: https://git.openjdk.org/shenandoah/pull/549