Revert "Removed an unused field, literally. :)"

This reverts commit 3b863cc6ff.

src/AST/Function.cs

@@ -248,6 +248,11 @@ namespace CppSharp.AST
         public string Signature { get; set; }
         public string Body { get; set; }
 
+        /// <summary>
+        /// Field associated in case of synthetized field acessors.
+        /// </summary>
+        public Field Field { get; set; }
+
         public override T Visit<T>(IDeclVisitor<T> visitor)
         {
             return visitor.VisitFunctionDecl(this);