RFR: 8358340: Support CDS heap archive with Generational Shenandoah

[Aleksey Shipilev]

Now that [JDK-8364111](https://bugs.openjdk.org/browse/JDK-8364111) is integrated, Generational Shenandoah can support CDS heap loads. This also allows Generational Shenandoah to work well with Leyden/AOT. We allocate things in young regions, and there is nothing else in the heap, so no card table updates are necessary.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah` 
 - [x] Linux x86_64 server fastdebug, `runtime/cds` with `-XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational`
 - [x] Linux x86_64 server fastdebug, `runtime/cds` with `-XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational -XX:+ShenandoahVerify`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/25597/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25597&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358340
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25597.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25597/head:pull/25597

PR: https://git.openjdk.org/jdk/pull/25597