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

Author: parkera

stdlib/public/SDK/Foundation/Measurement.swift

@@ -296,3 +296,15 @@ extension Measurement : _ObjectiveCBridgeable {
         return Measurement(value: source!.doubleValue, unit: u)
     }
 }
+
+// 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 {
+    public func string<UnitType: Unit>(from measurement: Measurement<UnitType>) -> String {
+        if let result = string(for: measurement) {
+            return result
+        } else {
+            return ""
+        }
+    }
+}

test/1_stdlib/TestMeasurement.swift

@@ -117,18 +117,25 @@ class TestMeasurement : TestMeasurementSuper {
         expectEqual(MyDimensionalUnit.unitA, MyDimensionalUnit.unitA)
         expectTrue(MyDimensionalUnit.unitA == MyDimensionalUnit.unitA)
     }
+    
+    func testMeasurementFormatter() {
+        let formatter = MeasurementFormatter()
+        let measurement = Measurement(value: 100, unit: UnitLength.kilometers)
+        let result = formatter.string(from: measurement)
+        
+        // Just make sure we get a result at all here
+        expectFalse(result.isEmpty)
+    }
 }
 
 #if !FOUNDATION_XCTEST
 if #available(OSX 10.12, iOS 10.0, tvOS 10.0, watchOS 3.0, *) {
-    // Temporarily disabled (26345468)
-    /*
     var MeasurementTests = TestSuite("TestMeasurement")
     MeasurementTests.test("testBasicConstruction") { TestMeasurement().testBasicConstruction() }
     MeasurementTests.test("testConversion") { TestMeasurement().testConversion() }
     MeasurementTests.test("testOperators") { TestMeasurement().testOperators() }
     MeasurementTests.test("testUnits") { TestMeasurement().testUnits() }
+    MeasurementTests.test("testMeasurementFormatter") { TestMeasurement().testMeasurementFormatter() }
     runAllTests()
-    */
 }
 #endif