RFR: 8348171: Refactor GenerationCounters and its subclasses [v3]

[Thomas Schatzl]

On Thu, 30 Jan 2025 14:59:04 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> 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 four commits:
> 
>  - review
>  - Merge branch 'master' into gen-counter
>  - merge
>  - gen-counter

The `VirtualSpace` forward declaration in `generationCounters.hpp` also seems to be unnecessary.

And there is an extra line between the last method declaration and the closing bracket of the `GenerationCounters` class.

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PR Comment: https://git.openjdk.org/jdk/pull/23209#issuecomment-2626857362