Add doc for _LIBCPP_ENABLE_CXX17_DISABLED_AUTO_PTR and make it work under _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@295407 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed