RFR: 8341859: Optimize ClassFile Benchmark Write

[Shaojin Wen]

Cache method names to reduce the overhead of using StringBuilder to construct method names, which will make the performance test results more stable.

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Commit messages:
 - cache methodName

Changes: https://git.openjdk.org/jdk/pull/21425/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21425&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341859
  Stats: 13 lines in 1 file changed: 9 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21425.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21425/head:pull/21425

PR: https://git.openjdk.org/jdk/pull/21425