RFR: 8333229: Parallel: Rename ParMarkBitMap::_region_start to _heap_start [v2]

[Thomas Schatzl]

On Mon, 3 Jun 2024 13:42:25 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Trivial renaming to better capture that this class corresponds to the whole heap mem-region.
>
> 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:
> 
>  - comment
>  - pgc-rename-heap

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/19468#pullrequestreview-2093925578