Add availability indicator to XMLElement to indicate correct function.

The implementation of setAttributesAs was deprecated bfeore the API was
copied over to the Linux implementation, and won't be implemented. By
adding an availability marker coupled with a new name, we allow for a
fix-it to run and use the new name in future compiled code.

Author: alblue

Foundation/NSXMLElement.swift

@@ -327,9 +327,10 @@ open class XMLElement: XMLNode {
 
 extension XMLElement {
     /*!
-        @method setAttributesAsDictionary:
+        @method setAttributesAs:
         @abstract Set the attributes base on a name-value dictionary.
-        @discussion This method is deprecated and does not function correctly. Use -setAttributesWithDictionary: instead.
+        @discussion This method is deprecated and does not function correctly. Use -setAttributesWith: instead.
      */
+    @available(*, unavailable, renamed:"setAttributesWith")
     public func setAttributesAs(_ attributes: [NSObject : AnyObject]) { NSUnimplemented() }
 }