RFR: 8299879: CollectedHeap hierarchy should use override

[Erik Österlund]

The CollectedHeap class declares a bunch of virtual methods that are overridden in its subclasses. Now that we can, we should convert this code to use "override" consistently, to help making the code more robust.

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Commit messages:
 - 8299879: CollectedHeap hierarchy should use override

Changes: https://git.openjdk.org/jdk/pull/11937/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11937&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299879
  Stats: 235 lines in 6 files changed: 2 ins; 5 del; 228 mod
  Patch: https://git.openjdk.org/jdk/pull/11937.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11937/head:pull/11937

PR: https://git.openjdk.org/jdk/pull/11937