RFR: 8336856: Optimize String Concat [v26]

[Shaojin Wen]

> This PR implements the same algorithm as the current generateMHInlineCopy based on bytecode to improve startup performance.

Shaojin Wen has updated the pull request incrementally with 14 additional commits since the last revision:

 - Merge pull request #9 from cl4es/pr_20273_inject_cache
   
   Cached prototype
 - Updated comment
 - Redo concatArgs
 - Shift constructor logic to shared superclass
 - merge
 - Remove defensive cloning (hidden class, trusted caller)
 - Rename
 - Mark all fields as @Stable
 - Simplify, promote byte and short arguments to int early
 - Weak -> Soft reference the constructor+handle pair
 - ... and 4 more: https://git.openjdk.org/jdk/compare/b7e1f524...c91b82a8

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/b7e1f524..c91b82a8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=25
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=24-25

  Stats: 262 lines in 2 files changed: 143 ins; 66 del; 53 mod
  Patch: https://git.openjdk.org/jdk/pull/20273.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20273/head:pull/20273

PR: https://git.openjdk.org/jdk/pull/20273