RFR: 8302344: Compiler Implementation for Unnamed patterns and variables (Preview) [v7]

[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 30 commits:

 - Merge branch 'JDK-8300543' into unnamed
 - Address comments around javax.lang
 - Fix field error detection
 - Update test files with JDK 8307007 bug id
 - Address javax.lang.model comments
 - Introduce new flag to track unnamed variables
   
   - Return underscore for Tree API and empty name for javax.lang.model.element.VariableElement.getSimpleName()
 - Fix indentation in JavacParser
 - Merge branch 'JDK-8300543' into unnamed
 - Fix top level type pattern with underscore
 - Introduce source in MemberEnter (javadoc uses MemberEnter)
 - ... and 20 more: https://git.openjdk.org/jdk/compare/3b3bcbcc...e39dd6dd

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Changes: https://git.openjdk.org/jdk/pull/13528/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=06
  Stats: 1132 lines in 44 files changed: 945 ins; 60 del; 127 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