[clang-format] Fix a clang-tidy warning, NFC

llvm-svn: 306409
diff --git a/clang/lib/Format/NamespaceEndCommentsFixer.cpp b/clang/lib/Format/NamespaceEndCommentsFixer.cpp
index 1bbb41f..85b70b8 100644
--- a/clang/lib/Format/NamespaceEndCommentsFixer.cpp
+++ b/clang/lib/Format/NamespaceEndCommentsFixer.cpp
@@ -174,7 +174,7 @@
         AllNamespaceNames = "::" + NamespaceName + AllNamespaceNames;
         continue;
       }
-      NamespaceName += std::move(AllNamespaceNames);
+      NamespaceName += AllNamespaceNames;
       CompactedNamespacesCount = 0;
       AllNamespaceNames = std::string();
     }