RFR: 8339127: GenShen: Restore completed mark context assertion during class unloading

[Kelvin Nilsen]

On Tue, 27 Aug 2024 21:25:09 GMT, William Kemper <wkemper at openjdk.org> wrote:

> We only unload classes during a global GC, so it is safe to assert that marking is complete at this point.
> 
> ### Testing
> GHA, internal pipelines

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/488#pullrequestreview-2266899992