Yes, it's just like calling an API supported by the SDK but not the deployment target--you must be in an availability context that lets you use the feature, otherwise you get an error at compile time for literals. For the initializer from String, it will throw when run with a standard library that does not have the feature. @Michael_Ilseman has been working through the details of how this all happens, mechanically.
For simple experiments, you can set DYLD_LIBRARY_PATH to point into the toolchain runtime directory, but be aware that this gives you a stdlib that isn't quite in-sync with the rest of the OS libraries, so you may see some errors for more complex programs that interact with other runtime libraries.
I think that you can probably work around this for more sophisticated experiments by using the toolchain compiler, but including the appropriate string-processing branch as a package dependency so that it shadows the toolchain-provided APIs and is used at runtime.