Ah, that's a good point. The feature is on the master branch; it's not yet on swift-4.2-branch.
If you're looking to backport these conditional conformances to 4.1, please be aware that (1) doing so will cause source compatibility issues down the road, and (2) the implementation you'll find on master uses internal stdlib interfaces that aren't available in 4.1. Earlier commits from the PR above have 4.1-compatible implementations, but they're still using using undocumented interfaces. Simply copying those into your own codebase would be a bad idea.
In general, only the standard library is allowed to conform stdlib types to stdlib protocols. (This rule isn't currently enforced by the language, but consider what would happen if two modules in the same app happened to define the same conformance.)