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
diff --git a/include/__config b/include/__config
index fd1dc86..7077f58 100644
--- a/include/__config
+++ b/include/__config
@@ -1057,6 +1057,7 @@
 #endif
 
 #if defined(_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES)
+# define _LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR
 # define _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS
 #endif // _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES