[1.0][benchmark] Update std::function syntax for current C++

[lorentey]

C++17 has removed std::unary_function. Migrate to using std::function.

(This may break our ability to build benchmarks with older clang/libc++ releases. That is fine.)

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.

[lorentey]

@swift-ci test