RFR: 8330694: Rename 'HeapRegion' to 'G1HeapRegion' [v7]

[Lei Zaakjyu]

> follow up 8267941

Lei Zaakjyu has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - rename
 - Merge branch 'master' of https://git.openjdk.org/jdk into JDK-8330694
 - review
 - Merge branch 'master' into JDK-8330694
 - fix indentation
 - also tidy up
 - tidy up
 - rename

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Changes: https://git.openjdk.org/jdk/pull/18871/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18871&range=06
  Stats: 999 lines in 124 files changed: 1 ins; 4 del; 994 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