RFR: 8302344: Compiler Implementation for Unnamed patterns and variables [v2]

[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:

  Introduce new flag to track unnamed variables
  
  - Return underscore for Tree API and empty name for javax.lang.model.element.VariableElement.getSimpleName()

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13528/files
  - new: https://git.openjdk.org/jdk/pull/13528/files/6e3f495e..624d3f4f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=00-01

  Stats: 308 lines in 16 files changed: 278 ins; 10 del; 20 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