Make OrderedSet.firstIndex ~4% faster by reducing ARC traffic

[dnadoba]

OrderedSet.firstIndex(of:) runs into some retain/release overhead that make up ~4% of its runtime.
Adding @_effects(releasenone) get's rid of this.

I have personally never used @_effects(releasenone) before but I have seen it used throughout this project. If I understand its behavior correctly it should be safe to use it here but I'm not certain.

Tested with

• Apple Swift version 6.1 (swiftlang-6.1.0.1.51 clang-1600.3.2.3)
• DEVELOPMENT-SNAPSHOT-2025-02-06-a (6.2.20250206101)

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.