RFR: 8369068: GenShen: Generations still aren't reconciled assertion failure

[William Kemper]

There are certain code paths in Shenandoah's generational mode that need to _know_ which generation is being collected, but it is not possible to pass this information on the stack (barriers, for example). To address this, we introduced an `_active_generation` member in `ShenandoahHeap`. Over time, the usage of this field grew beyond its intended purpose and we began to have issues where not all threads would see a consistent value for this field. To address _this_ issue, we added another field `_gc_generation` which was only meant to be used by GC threads. At this point, we have three different ways to determine which generation is being collected: the _active_ generation, the _gc_ generation, and the usual function parameters and member fields of the gc components. This PR removes `_gc_generation` and reduces the use of `_active_generation` to only those places where it is not possible to get this information from other means (barriers, mostly). All GC components that can have this
  information passed through function calls, now do so.

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Commit messages:
 - Merge remote-tracking branch 'jdk/master' into reduce-gc_generation-usage
 - Remove _gc_generation from ShenandoahHeap
 - Little cleanup, remove one active generation usage
 - Merge remote-tracking branch 'jdk/master' into reduce-gc_generation-usage
 - Finish removing usages of gc_generation, start on reducing usages of active_generation
 - Fix build
 - Use existing _generation field instead of Heap::_gc_generation where possible
 - Only shenandoah vm operations participate in active/gc generation scheme

Changes: https://git.openjdk.org/jdk/pull/27703/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27703&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369068
  Stats: 495 lines in 32 files changed: 119 ins; 149 del; 227 mod
  Patch: https://git.openjdk.org/jdk/pull/27703.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27703/head:pull/27703

PR: https://git.openjdk.org/jdk/pull/27703