RFR: 8339640: Reduce construction overheads in StringConcatFactory$InlineHiddenClassStrategy

[Claes Redestad]

Using the trusted `invokeBasic` method streamlines invokers and avoids spinning up of some type conversion handles. This reduces classes generated in a few scalability stress tests.

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

Commit messages:
 - Use invokeBasic to construct StringConcat instances in StringConcatFactory

Changes: https://git.openjdk.org/jdk/pull/20884/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20884&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339640
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20884.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20884/head:pull/20884

PR: https://git.openjdk.org/jdk/pull/20884