Refactored ConvertMemberToExtension to re-use original declaration instead of regenerating it by text

#KT-4920 Fixed

idea/testData/intentions/declarations/convertMemberToExtension/varWithGetter.kt.after

@@ -3,9 +3,7 @@ class Owner {
 }
 
 var Owner.p: Int
-    get() {
-        return 1
-    }
+    get() { return 1 }
     set(value) {
         <caret><selection>throw UnsupportedOperationException()</selection>
     }