RFR: 8315532: Compiler Implementation for Unnamed Variables and Patterns [v4]

[Aggelos Biboudis]

> This PR finalizes the feature of unnamed variables and patterns.

Aggelos Biboudis has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Add test with multi-catch
 - Merge branch 'master' into unnamed
 - Address review by @mcimadamore
 - Address review by @lahodaj
 - 8315532: Compiler Implementation for Unnamed Variables and Patterns

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15649/files
  - new: https://git.openjdk.org/jdk/pull/15649/files/630b128e..ccbfbd4f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15649&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15649&range=02-03

  Stats: 99666 lines in 2717 files changed: 41569 ins; 19343 del; 38754 mod
  Patch: https://git.openjdk.org/jdk/pull/15649.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15649/head:pull/15649

PR: https://git.openjdk.org/jdk/pull/15649