RFR: 8302344: Compiler Implementation for Unnamed patterns and variables (Preview) [v8]

[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 around JavacParser (remove identOrFlagUnderscore)

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13528/files
  - new: https://git.openjdk.org/jdk/pull/13528/files/e39dd6dd..47d738bb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=06-07

  Stats: 39 lines in 1 file changed: 12 ins; 20 del; 7 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