RFR: 8276932: G1: Annotate methods with override explicitly in g1CollectedHeap.hpp

[Albert Mingkun Yang]

This PR consists of two commits: the first commit is generated using `clang-tidy --fix src/hotspot/share/gc/g1/g1CollectedHeap.hpp -checks=modernize-use-override -p build/linux-x64-debug`, and the second commit fixes some indentation issues.

Test: build

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Commit messages:
 - indent
 - override

Changes: https://git.openjdk.java.net/jdk/pull/6330/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6330&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276932
  Stats: 52 lines in 1 file changed: 0 ins; 0 del; 52 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6330.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6330/head:pull/6330

PR: https://git.openjdk.java.net/jdk/pull/6330