RFR: 8302344: Compiler Implementation for Unnamed patterns and variables (Preview) [v15]

[Aggelos Biboudis]

> This PR implements [JEP 443](https://openjdk.org/jeps/443), the preview feature for Unnamed Patterns and Variables in Java.
> 
> Draft Spec: https://cr.openjdk.org/~abimpoudis/unnamed/latest/

Aggelos Biboudis has updated the pull request incrementally with one additional commit since the last revision:

  Address comments
  
  - Annotate with Preview
  - Fix isUnnamed hierarchy
  - Flow names.empty for underscore in both model and API
  - Adjust pretty printing and annotation pretty printing
  - Fix guard bindings (great catch and thx @lahodaj!)
  - Fix TreeVisitor
  - Remove test with lambda which has a parse error from VarTree (names.empty is used to signal error)
  - Various adjustments
  
  Co-authored-by: Jan Lahoda <jan.lahoda at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13528/files
  - new: https://git.openjdk.org/jdk/pull/13528/files/888297ef..5d9a1ba7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=13-14

  Stats: 348 lines in 25 files changed: 189 ins; 102 del; 57 mod
  Patch: https://git.openjdk.org/jdk/pull/13528.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13528/head:pull/13528

PR: https://git.openjdk.org/jdk/pull/13528