RFR: 8264797: Do not include klassVtable.hpp from instanceKlass.hpp

[Ioi Lam]

instanceKlass.hpp is a popular header, included by 647 of about 1000 HotSpot .o files. It unnecessarily includes klassVtable.hpp (only forward class declarations are needed).

After refactoring, the number of .o files that includes klassVtable.hpp reduces from 647 to 132.

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Commit messages:
 - 8264797: Do not include klassVtable.hpp from instanceKlass.hpp

Changes: https://git.openjdk.java.net/jdk/pull/3360/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3360&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264797
  Stats: 6 lines in 5 files changed: 5 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3360.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3360/head:pull/3360

PR: https://git.openjdk.java.net/jdk/pull/3360