RFR: 8337168: Optimize LocalDateTime.toString [v3]

[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.

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  change description

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20321/files
  - new: https://git.openjdk.org/jdk/pull/20321/files/7b68b32b..e7c5bc69

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20321&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20321&range=01-02

  Stats: 19 lines in 2 files changed: 0 ins; 15 del; 4 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