RFR: 8338620: GenShen: Distinguish between young and old collector in GC configuration

[William Kemper]

Return `ShenandoahYoung` for `young_collector` under generational mode (continue returning `N/A` for other modes). Return `ShenandoahOld` for `old_collector` under generational mode (continue returning `Shenandoah` for other modes).

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into distinguish-young-old-gc
 - Add ShenandoahYoung and ShenandoahOld gc names

Changes: https://git.openjdk.org/shenandoah/pull/477/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=477&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338620
  Stats: 36 lines in 2 files changed: 22 ins; 14 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/477.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/477/head:pull/477

PR: https://git.openjdk.org/shenandoah/pull/477