[lworld] RFR: 8373912: [lworld] Small cleanups to InlineKlass

[Stefan Karlsson]

A few cleanups found while scanning over the InlineKlass files:

* Fix include list and forward declarations
* Move non-trivial implementation out of .hpp
* Remove copy-n-past functions from InstanceKlass
* Remove redundant CDS function overrides
* Deduplicate null marker address calculation
* Deduplicate allocate_instance
* Fix raw casts from oop 
* Tweak comments

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Commit messages:
 - 8373912: [lworld] Small cleanups to InlineKlass

Changes: https://git.openjdk.org/valhalla/pull/1818/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1818&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373912
  Stats: 75 lines in 5 files changed: 25 ins; 38 del; 12 mod
  Patch: https://git.openjdk.org/valhalla/pull/1818.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1818/head:pull/1818

PR: https://git.openjdk.org/valhalla/pull/1818