RFR: 8288537: Refactor hotspot/share/memory/iterator.hpp

[Ioi Lam]

HotSpot build time has regressed quite a bit since JDK 17 (1m46s vs 2m20s on my machine). Now it's time to do some header file cleanup.

iterator.hpp contains a hodge-podge of unrelated classes. Some of these classes have dependencies on other headers (e.g., `Devirtualizer` depends on bitMap.hpp) that slow down C++ compilation.

This patch moves two infrequently used classes, `Devirtualizer` and `SerializeClosure`, to their own header files. This reduces the total number of C++ lines compiled for libjvm.so for about 1%.

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Commit messages:
 - fixed Shenandoah build
 - 8288537: Refactor hotspot/share/memory/iterator.hpp

Changes: https://git.openjdk.org/jdk/pull/9176/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9176&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288537
  Stats: 568 lines in 28 files changed: 337 ins; 214 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/9176.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9176/head:pull/9176

PR: https://git.openjdk.org/jdk/pull/9176