RFR: 8302344: Compiler Implementation for Unnamed patterns and variables (Preview) [v24]

[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 with a new target base due to a merge or a rebase. The pull request now contains 49 commits:

 - Merge branch 'JDK-8300543' into unnamed
   
   # Conflicts:
   #	src/java.base/share/classes/jdk/internal/javac/PreviewFeature.java
   #	src/jdk.compiler/share/classes/com/sun/source/tree/TreeVisitor.java
   #	src/jdk.compiler/share/classes/com/sun/tools/javac/code/Preview.java
 - Cleanup
 - Add at since tag
 - Remove Elements.isUnamed(VariableElement) given the presence of VariableElement.isUnamed
 - Doc adjustments
 - Merge branch 'JDK-8300543' into unnamed
 - Reintroduce isUnnamed from VarSymbol with different name
 - Revert "Remove isUnnamed from VarSymbol"
   
   This reverts commit a46b674a5077da603c400f2005cf4a7a48d104b2.
 - Remove isUnnamed from VarSymbol
 - Remove @Override from VarSymbol.isUnnamed
 - ... and 39 more: https://git.openjdk.org/jdk/compare/69ba2a57...94997d16

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Changes: https://git.openjdk.org/jdk/pull/13528/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=23
  Stats: 1480 lines in 46 files changed: 1159 ins; 144 del; 177 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