RFR: JDK-8329624: Add visitors for preview language features [v2]

[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.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Add informative links.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18609/files
  - new: https://git.openjdk.org/jdk/pull/18609/files/45bbc292..5fc414bc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18609&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18609&range=00-01

  Stats: 36 lines in 10 files changed: 22 ins; 2 del; 12 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