RFR: 8332548: GenShen: Factor generational mode out of gc helpers [v2]

[William Kemper]

> Some of the generational mode support in `shenandoahConcurrentGC.cpp` and `shenandoahDegeneratedGC.cpp` can be factored into generational mode specific files and coalesced under fewer generational mode checks.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Templatize evact update task and closure to prevent excessive mode checks
 - Use descriptive style in comments for consistency

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/436/files
  - new: https://git.openjdk.org/shenandoah/pull/436/files/31ffa8f6..3c5cf2e1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=436&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=436&range=00-01

  Stats: 30 lines in 2 files changed: 7 ins; 6 del; 17 mod
  Patch: https://git.openjdk.org/shenandoah/pull/436.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/436/head:pull/436

PR: https://git.openjdk.org/shenandoah/pull/436