RFR: 8302344: Compiler Implementation for Unnamed patterns and variables (Preview) [v29]

[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 55 commits:

 - Merge branch 'JDK-8300543' into unnamed
   
   # Conflicts:
   #	src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java
 - Reduce the error message
 - Improve error reporting
 - Address review around `LambdaClassifier`
 - Simplify `LambdaClassifier`
   
   Co-authored-by: Maurizio Cimadamore <maurizio.cimadamore at oracle.com>
 - Fix compiler bugs
   
   - Fix skipped errors when using unnamed locals without initializer
   - Fix lambdas with mix unnamed and named parameters
 - 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
 - ... and 45 more: https://git.openjdk.org/jdk/compare/924ca58d...9502c1e9

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Changes: https://git.openjdk.org/jdk/pull/13528/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=28
  Stats: 1765 lines in 53 files changed: 1313 ins; 153 del; 299 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