More list debug mode tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178873 91177308-0d34-0410-b5e6-96231b3b80d8
Fix bug in __libcpp_db::__iterator_copy.  Add debug test for swaping lists.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178892 91177308-0d34-0410-b5e6-96231b3b80d8