RFR: 8330694: Rename 'HeapRegion' to 'G1HeapRegion'

[Lei Zaakjyu]

follow up 8267941

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Commit messages:
 - rename

Changes: https://git.openjdk.org/jdk/pull/18871/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18871&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330694
  Stats: 978 lines in 124 files changed: 0 ins; 0 del; 978 mod
  Patch: https://git.openjdk.org/jdk/pull/18871.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18871/head:pull/18871

PR: https://git.openjdk.org/jdk/pull/18871