RFR: 8337168: Optimize LocalDateTime.toString [v2]

[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 two additional commits since the last revision:

 - add comments
 - reduce change

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20321/files
  - new: https://git.openjdk.org/jdk/pull/20321/files/c03ed26f..7b68b32b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20321&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20321&range=00-01

  Stats: 24 lines in 2 files changed: 23 ins; 0 del; 1 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