RFR: 8271623: Omit enclosing instance fields from inner classes that don't use it [v3]

[Liam Miller-Cushon]

> This change omits the synthetic `this$0` field from inner classes that do not access any enclosing instance state.

Liam Miller-Cushon has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Enable enclosing instance optimization for --release 18 and newer
 - Add a test case with nested inner classes
 - 8271623: Omit enclosing instance fields from inner classes that don't use it

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Changes: https://git.openjdk.java.net/jdk/pull/4966/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4966&range=02
  Stats: 396 lines in 16 files changed: 357 ins; 12 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4966.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4966/head:pull/4966

PR: https://git.openjdk.java.net/jdk/pull/4966