swift234

Add a workaround for MeasurementFormatter so it can use Measurement

Due to current language limitations (26607639), MeasurementFormatter's
stringFromMeasurement: is esentially useless when imported into Swift.
This workaround allows it to work as expected.

<rdar://problem/27173952> Workaround: MeasurementFormatter.string(from:
Measurement<Unit>) doesn't recognize subclasses of Unit