RFR: 8339875: MethodTypeDesc to reuse descriptor string from MethodType

[Chen Liang]

A micro-optimization by sharing compatible String objects from MethodType (which itself is already deduplicated in a cache) to the generated MethodTypeDesc. This very slightly improves hashing performance by avoiding some string creation, hashing, and fast path equality via identity.

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Commit messages:
 - 8339875: MethodTypeDesc to reuse descriptor string from MethodType

Changes: https://git.openjdk.org/jdk/pull/20942/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20942&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339875
  Stats: 49 lines in 7 files changed: 24 ins; 12 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/20942.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20942/head:pull/20942

PR: https://git.openjdk.org/jdk/pull/20942