RFR: 8325671: Shenandoah: Introduce a ShenandoahGenerationType and templatize certain marking closures with it [v4]

[Y. Srinivas Ramakrishna]

> 8325671: Shenandoah: Introduce a ShenandoahGenerationType and templatize certain marking closures with it

Y. Srinivas Ramakrishna has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge branch 'master' into generation_type
 - Merge branch 'master' into generation_type
 - Missing #include of ShenandoahGenerationType (albeit satisfied transitively via other include).
 - Merge branch 'master' into generation_type
 - Introduce ShenandoahGenerationType and templatize most closures with it.
   The template expands for only the NON_GEN type for the non-generational
   version of Shenandoah currently, and will in the future accomodate
   Generational Shenandoah.

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Changes: https://git.openjdk.org/jdk/pull/17815/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17815&range=03
  Stats: 120 lines in 9 files changed: 69 ins; 0 del; 51 mod
  Patch: https://git.openjdk.org/jdk/pull/17815.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17815/head:pull/17815

PR: https://git.openjdk.org/jdk/pull/17815