RFR: 8260736: Shenandoah: Cleanup includes in ShenandoahGC and families

[Zhengyu Gu]

8260736: Shenandoah: Cleanup includes in ShenandoahGC and families

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Commit messages:
 - update
 - Merge branch 'master' into JDK-8260736-cleanup-includes-gc
 - update
 - init

Changes: https://git.openjdk.java.net/jdk/pull/2339/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2339&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260736
  Stats: 37 lines in 10 files changed: 2 ins; 30 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2339.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2339/head:pull/2339

PR: https://git.openjdk.java.net/jdk/pull/2339