RFR: 8336856: Efficient hidden class-based string concatenation strategy [v48]

[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 one additional commit since the last revision:

  add jtreg HiddenClassUnloading

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/663d6224..68506248

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=47
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=46-47

  Stats: 104 lines in 1 file changed: 104 ins; 0 del; 0 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