Highlight async/await

Author: taku0

swift-mode-font-lock.el

@@ -550,7 +550,7 @@ Return nil otherwise."
 
 (defconst swift-mode:expression-keywords
   '("as" "catch" "dynamicType" "is" "rethrows" "super" "self" "Self" "throws"
-    "throw" "try")
+    "throw" "try" "async" "await")
   "Keywords used in expressions and types.
 
 Excludes true, false, and keywords begin with a number sign.")