Add missing std::move call

This should fix
http://green.lab.llvm.org/green/job/clang-stage1-cmake-RA-incremental_build/41578/

llvm-svn: 311656

clang/lib/Tooling/Refactoring/ASTSelection.cpp

@@ -42,7 +42,7 @@ public:
     SelectionStack.pop_back();
     if (Result.Children.empty())
       return None;
-    return Result;
+    return std::move(Result);
   }
 
   bool TraverseDecl(Decl *D) {