RFR: 8302344: Compiler Implementation for Unnamed patterns and variables

[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/

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Depends on: https://git.openjdk.org/jdk/pull/13074

Commit messages:
 - Fix indentation in JavacParser
 - Merge branch 'JDK-8300543' into unnamed
 - Fix top level type pattern with underscore
 - Introduce source in MemberEnter (javadoc uses MemberEnter)
 - Merge branch 'JDK-8300543' into unnamed
 - Fix parsePattern after merge
 - Fix
 - Implement cases with multiple unnamed patterns and guards
 - WIP: guards
 - Merge branch 'JDK-8300543' into unnamed
 - ... and 14 more: https://git.openjdk.org/jdk/compare/35f51562...6e3f495e

Changes: https://git.openjdk.org/jdk/pull/13528/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13528&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302344
  Stats: 767 lines in 32 files changed: 596 ins; 52 del; 119 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