RFR: 8339115: Rename TypeKind enum constants to follow code style

[Chen Liang]

`TypeKind` enum constants are named in wrong code style; correct them before finalization.

Also improved `TypeKind` specification to talk about not mentioned `returnType`, `void`, and subword types being erased to int (and how). See the associated CSR too.

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Commit messages:
 - 8339115: Rename TypeKind enum constants to follow code style

Changes: https://git.openjdk.org/jdk/pull/20737/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20737&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339115
  Stats: 664 lines in 31 files changed: 70 ins; 14 del; 580 mod
  Patch: https://git.openjdk.org/jdk/pull/20737.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20737/head:pull/20737

PR: https://git.openjdk.org/jdk/pull/20737