RFR: 8271623: Omit enclosing instance fields from inner classes that don't use it [v2]

[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 incrementally with one additional commit since the last revision:

  Add a test case with nested inner classes

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4966/files
  - new: https://git.openjdk.java.net/jdk/pull/4966/files/0ff3a3b9..3db4488c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4966&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4966&range=00-01

  Stats: 31 lines in 1 file changed: 23 ins; 0 del; 8 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