[foreign-abi] RFR: 8254156: Simplify ABI classification logic

[Maurizio Cimadamore]

This patch reverts recent changes to introduce a public CValueLayout class; the realization here is that, after all,
CValueLayout exposes a public `Type` enum which is used by ABI classification. So, instead of using subclassing (which
is messy and tedious with immutable data structures with covariant overrides), let's just double down on layout
attributes, get rid of CValueLayout and simply expose the Type enum (now renamed to TypeKind).

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Commit messages:
 - Address review comments
 - Revert ValueLayout subclassing

Changes: https://git.openjdk.java.net/panama-foreign/pull/371/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=371&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254156
  Stats: 318 lines in 9 files changed: 43 ins; 153 del; 122 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/371.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/371/head:pull/371

PR: https://git.openjdk.java.net/panama-foreign/pull/371