RFR: 8336856: Efficient hidden class-based string concatenation strategy [v50]

[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 three additional commits since the last revision:

 - Simplify HiddenClassUnloading
 - Remove unnecessary loops
 - fix build error

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/32334915..babc76e1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=49
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=48-49

  Stats: 51 lines in 2 files changed: 4 ins; 35 del; 12 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