[ASTWalker] NFC: Rename SkipChildren -> SkipNode

This better describes what the action currently
does, and allows us to re-introduce `SkipChildren`
with the correct behavior.

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1608,7 +1608,7 @@ private:
             Elem.walk(*this);
           }
         }
-        return Action::SkipChildren();
+        return Action::SkipNode();
       }
       return Action::Continue();
     }