Don't neglect to "return *this".

llvm-svn: 165860
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 88db3171dd5dcc54e3890b6f823c60831fc520a5
diff --git a/include/string b/include/string
index 7c022dc..89a00e5 100644
--- a/include/string
+++ b/include/string
@@ -2208,6 +2208,7 @@
     clear();
     for (; __first != __last; ++__first)
         push_back(*__first);
+    return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>