RFR: 8325671: Shenandoah: Introduce a ShenandoahGenerationType and templatize certain marking closures with it [v2]

[Y. Srinivas Ramakrishna]

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

Y. Srinivas Ramakrishna has updated the pull request incrementally with one additional commit since the last revision:

  Missing #include of ShenandoahGenerationType (albeit satisfied transitively via other include).

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17815/files
  - new: https://git.openjdk.org/jdk/pull/17815/files/25397d03..04c32336

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17815&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17815&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 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