RFR: 8342588: Shenandoah: Pull shared closures together in one header

[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`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21579/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21579&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342588
  Stats: 448 lines in 16 files changed: 188 ins; 234 del; 26 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