Omit needless words: allow removal of words following a gerund.

Identify gerunds by stripping off the "ing" and looking for a
verb. This lets us transform, e.g., "stringByAppendingString" to
"stringByAppending", since "append" is a verb.

Swift SVN r31660

include/swift/Basic/StringExtras.h

@@ -40,7 +40,8 @@ namespace swift {
   enum class PartOfSpeech {
     Unknown,
     Preposition,
-    Verb
+    Verb,
+    Gerund,
   };
 
   /// Determine the part of speech for the given word.