Reimplement _specialize(_:for:) for the 5.9 stdlib

[lorentey]

This was introduced in the standard library in Swift 5.10, which postdates our current minimum toolchain version.

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.