[lworld] RFR: 8355227: [lworld] Clarify preview API status for some vallhalla APIs

[Chen Liang]

Per [JEP 12](https://openjdk.org/jeps/12):

> In order for frameworks to deliver early support for a preview language feature, frameworks should utilize the reflective preview APIs that are co-developed with it. This will allow frameworks to keep up when programs are written to use the permanent version of the language feature.

According to these rules, the `Objects` methods and the `IdentityException` class themselves must be reflective preview APIs: programs compiled without preview feature dependencies that are running with preview features enabled should be able to access these APIs, as at run-time, they may receive value objects and `IdentityException` thrown by the JVM.

The only remaining essential preview APIs are the constructors of `IdentityException`: non-preview programs have no reason to construct them.

In addition, fixed a few JEP feature reference: notably, add reference to [Strict Fields JEP](https://openjdk.org/jeps/8350458).

Also, made ClassFile API new models reflective preview APIs - class file processors wish to process structures from preview class files even if the current runtime is not in preview. This is already happening to javac and javap.

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Commit messages:
 - 8355227: [lworld] Clarify preview API status for some vallhalla APIs

Changes: https://git.openjdk.org/valhalla/pull/1439/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1439&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355227
  Stats: 19 lines in 6 files changed: 12 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/valhalla/pull/1439.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1439/head:pull/1439

PR: https://git.openjdk.org/valhalla/pull/1439