RFR: 8330694: Rename 'HeapRegion' to 'G1HeapRegion' [v12]

[Lei Zaakjyu]

> follow up 8267941

Lei Zaakjyu has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  restore

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18871/files
  - new: https://git.openjdk.org/jdk/pull/18871/files/dafdc775..c240897a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18871&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18871&range=10-11

  Stats: 19 lines in 6 files changed: 0 ins; 0 del; 19 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