Fix -Wpessimizing-move warning by remove the call to std::move.

llvm-svn: 236265
diff --git a/libcxx/src/system_error.cpp b/libcxx/src/system_error.cpp
index c032fc0..18f668f 100644
--- a/libcxx/src/system_error.cpp
+++ b/libcxx/src/system_error.cpp
@@ -152,7 +152,7 @@
             what_arg += ": ";
         what_arg += ec.message();
     }
-    return _VSTD::move(what_arg);
+    return what_arg;
 }
 
 system_error::system_error(error_code ec, const string& what_arg)