Foundation: add commented out custom AnyHashable representation to Measurement

It is disabled because bridging of Measurement instances does not actually work now.

Author: gribozavr

stdlib/public/SDK/Foundation/Measurement.swift

@@ -288,6 +288,18 @@ extension Measurement : _ObjectiveCBridgeable {
     }
 }
 
+/*
+FIXME(id-as-any): can't write this code because of:
+<rdar://problem/27539951> "unhandled generic bridged type" when bridging NSMeasurement
+
+@available(OSX 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
+extension NSMeasurement : _HasCustomAnyHashableRepresentation {
+    public func _toCustomAnyHashable() -> AnyHashable? {
+        return AnyHashable(self as Measurement)
+    }
+}
+*/
+
 // This workaround is required for the time being, because Swift doesn't support covariance for Measurement (26607639)
 @available(OSX 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
 extension MeasurementFormatter {