I believe this finishes up debug mode for list.  The testing is a little weak, but I believe all of the functionality is there.  Certainly enough for people to checkout and start beating up on.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@179632 91177308-0d34-0410-b5e6-96231b3b80d8