RFR: JDK-8329624: Add visitors for preview language features

[Joe Darcy]

When new language features are added, the javax.lang.model may need to be updated. For certain classes of features, the API update includes introducing a new set of concrete visitors to handle the language feature.

The API scaffolding to support the new feature tends to be considerably larger than the API specifically for the new feature.

To aid work in progress (such as https://github.com/openjdk/jdk/pull/18509) and anticipated in the future, I think it would be helpful to introduce a persistent set of preview visitors independent of any particular language change.

-------------

Commit messages:
 - Remove accidentally added file.
 - JDK-8329624: Add visitors for preview language features

Changes: https://git.openjdk.org/jdk/pull/18609/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18609&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329624
  Stats: 724 lines in 11 files changed: 715 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18609.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18609/head:pull/18609

PR: https://git.openjdk.org/jdk/pull/18609