[libc++] Refactor node creation and destruction in list and forward_list

This removes a lot of code duplication, makes the code simpler and
prepares the terrain for https://reviews.llvm.org/D101206, which will
fix some UB in the node-based containers.

This also allows removing the dependency of list and forward_list on
unique_ptr by using __allocation_guard instead.

Author: ldionne

libcxx/include/chrono

@@ -846,6 +846,8 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 #  include <bit>
 #  include <concepts>
 #  include <cstring>
+#  include <forward_list>
+#  include <string>
 #endif
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER == 20