RFR: 8302344: Compiler Implementation for Unnamed patterns and variables (Preview) [v18]

[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 two additional commits since the last revision:

 - Reintroduce isUnnamed from VarSymbol with different name
 - Revert "Remove isUnnamed from VarSymbol"
   
   This reverts commit a46b674a5077da603c400f2005cf4a7a48d104b2.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13528/files
  - new: https://git.openjdk.org/jdk/pull/13528/files/a46b674a..8def897a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=17
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=16-17

  Stats: 15 lines in 7 files changed: 4 ins; 5 del; 6 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