Apply a similar fix to <forward_list> as I did for <list> in r215210. Again, thanks to Ion Gaztañaga for noticing this problem w.r.t LWG#526

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215213 91177308-0d34-0410-b5e6-96231b3b80d8