RFR: 8376756: GenShen: Improve encapsulation of generational collection set choosing [v2]

[William Kemper]

> Choosing the collection set for Shenandoah's generational mode is somewhat more complicated than the non-generational modes. The logic is spread out in unnatural places which compounds the complexity. This change was spun off from https://github.com/openjdk/jdk/pull/27632. There is more to come, but I'm breaking the changes up over several PRs to facilitate reviewing.
> 
> Description of changes:
> 
> The methods:
> * `ShGeneration::compute_evacuation_budgets`
> * `ShGeneration::select_aged_regions`
> * `ShGeneration::adjust_evacuation-budgets`
> 
> Were all moved into `ShGenerationalHeuristics`. The signature of `ShHeuritics::choose_collection_set` is restored to return `void` instead of returning a `size_t` which represents the number of old regions to be added to top off the collection set. The one heuristic that returned a non-zero value for this (`ShYoungHeuristics`) now carries it in a member variable.
> 
> Redundant keyword modifiers have been removed from the headers touched here.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Fix zero build

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/29496/files
  - new: https://git.openjdk.org/jdk/pull/29496/files/52063e35..cf28eff5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29496&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29496&range=00-01

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29496.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29496/head:pull/29496

PR: https://git.openjdk.org/jdk/pull/29496