[stdlib] Drop unneeded protocol requirement

Conversion to cocoa is not a common requirement for all models of
_ArrayBufferType

Swift SVN r22947

stdlib/core/ArrayCast.swift

@@ -56,7 +56,7 @@ public func _arrayForceCast<SourceElement, TargetElement>(
       }
     }
     // This result has deferred element typechecking
-    return Array(_fromCocoaArray: source._asCocoaArray())
+    return Array(_fromCocoaArray: source._buffer._asCocoaArray())
     
   case (.Reference, .Explicit):
     let result: [TargetElement]? = _arrayConditionalBridgeElements(source)