RFR: 8344779: GenShen: Consolidate shared heap region closures into one header

[Kelvin Nilsen]

On Thu, 21 Nov 2024 19:25:27 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Move shared heap region closure implementations into one header, move non-shared implementations closer to their only usage.

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/549#pullrequestreview-2453040529