RFR: 8337168: Optimize LocalDateTime.toString

[Shaojin Wen]

The current LocalDateTime.toString implementation concatenates the toString results of date and time, which can be passed to StringBuilder to reduce additional object allocation and improve performance.

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Commit messages:
 - optimize LocalDateTime.toString, reduce StringBuilder allocate

Changes: https://git.openjdk.org/jdk/pull/20321/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20321&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337168
  Stats: 26 lines in 3 files changed: 15 ins; 3 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/20321.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20321/head:pull/20321

PR: https://git.openjdk.org/jdk/pull/20321