RFR: 8336856: Efficient hidden class-based string concatenation strategy [v49]

[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 four additional commits since the last revision:

 - Update src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
   
   Co-authored-by: Claes Redestad <claes.redestad at oracle.com>
 - Update src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
   
   Co-authored-by: Claes Redestad <claes.redestad at oracle.com>
 - Update test/jdk/java/lang/String/concat/HiddenClassUnloading.java
   
   Co-authored-by: Claes Redestad <claes.redestad at oracle.com>
 - Update src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
   
   Co-authored-by: Claes Redestad <claes.redestad at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/68506248..32334915

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=48
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=47-48

  Stats: 7 lines in 2 files changed: 1 ins; 3 del; 3 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