RFR: 8348171: Refactor GenerationCounters and its subclasses [v2]

[Albert Mingkun Yang]

> Simple refactoring of removing the use of `virtual` method and use concrete subclasses when needed.
> 
> Test: tier1-5

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - merge
 - gen-counter

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Changes: https://git.openjdk.org/jdk/pull/23209/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23209&range=01
  Stats: 194 lines in 16 files changed: 8 ins; 156 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/23209.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23209/head:pull/23209

PR: https://git.openjdk.org/jdk/pull/23209