RFR: 8339704: Refactor StringConcatHelper simpleConcat [v5]

[Shaojin Wen]

> The string concatenation of the java.base module is implemented using StringBuilder. By providing a series of concat methods in StringConcatHelper, it is used in the java.lang package to replace string concatenation.
> 
> These concat methods can also be exposed through JLA for use by other packages, such as java.lang.constant.
> 
> These concat methods can replace Concat1 and become part of StringConcatFactory#simpleConcat

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

  reduce changes

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20726/files
  - new: https://git.openjdk.org/jdk/pull/20726/files/202934ad..f63e6a7f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20726&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20726&range=03-04

  Stats: 5 lines in 1 file changed: 0 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20726.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20726/head:pull/20726

PR: https://git.openjdk.org/jdk/pull/20726