Don't neglect to "return *this".

llvm-svn: 165860
diff --git a/libcxx/include/string b/libcxx/include/string
index 7c022dc..89a00e5 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -2208,6 +2208,7 @@
     clear();
     for (; __first != __last; ++__first)
         push_back(*__first);
+    return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>