RFR: 8342588: Shenandoah: Pull shared closures together in one header [v4]

[Aleksey Shipilev]

> Currently we have shenandoahClosures.hpp and shenandoahOopClosures.hpp, each of which have some closure inside. There is no reason to have this separation, and we should really just merge everything in one header. I also did a bit of mechanical renames. There should be no semantic change with this patch.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into JDK-8342588-shenandoah-shared-closures
 - Merge branch 'master' into JDK-8342588-shenandoah-shared-closures
 - Fix Zero builds
 - Fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21579/files
  - new: https://git.openjdk.org/jdk/pull/21579/files/831372cf..ad14c6d0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21579&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21579&range=02-03

  Stats: 798 lines in 31 files changed: 553 ins; 132 del; 113 mod
  Patch: https://git.openjdk.org/jdk/pull/21579.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21579/head:pull/21579

PR: https://git.openjdk.org/jdk/pull/21579