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

[Liam Miller-Cushon]

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

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Commit messages:
 - 8271623: Omit enclosing instance fields from inner classes that don't use it

Changes: https://git.openjdk.java.net/jdk/pull/4966/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4966&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271623
  Stats: 261 lines in 14 files changed: 216 ins; 15 del; 30 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