RFR: 8336491: Unnecessary boxing conversions in void-returning lambdas

[Liam Miller-Cushon]

Please consider this possible fix for [JDK-8336491](https://bugs.openjdk.org/browse/JDK-8336491), which avoids generating an unnecessary boxing conversion for the result of void-returning expression lambdas.

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Commit messages:
 - 8336491: Unnecessary boxing conversions in void-returning lambdas

Changes: https://git.openjdk.org/jdk/pull/20222/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20222&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336491
  Stats: 65 lines in 2 files changed: 64 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20222.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20222/head:pull/20222

PR: https://git.openjdk.org/jdk/pull/20222