RFR: 8342588: Shenandoah: Pull shared closures together in one header [v2]

[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 incrementally with one additional commit since the last revision:

  Fix Zero builds

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21579/files
  - new: https://git.openjdk.org/jdk/pull/21579/files/cbc94ff4..c9af382c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21579&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21579&range=00-01

  Stats: 12 lines in 2 files changed: 7 ins; 3 del; 2 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