libstdc++: Fix fancy pointer support in linked lists [PR57272]

The union members I used in the new _Node types for fancy pointers only
work for value types that are trivially default constructible. This
change replaces the anonymous union with a named union so it can be
given a default constructor and destructor, to leave the variant member
uninitialized.

This also fixes the incorrect macro names in the alloc_ptr_ignored.cc
tests as pointed out by François, and fixes some std::list pointer
confusions that the fixed alloc_ptr_ignored.cc test revealed.

libstdc++-v3/ChangeLog:

	PR libstdc++/57272
	* include/bits/forward_list.h (__fwd_list::_Node): Add
	user-provided special member functions to union.
	* include/bits/stl_list.h (__list::_Node): Likewise.
	(_Node_base::_M_hook, _Node_base::swap): Use _M_base() instead
	of std::pointer_traits::pointer_to.
	(_Node_base::_M_transfer): Likewise. Add noexcept.
	(_List_base::_M_put_node): Use 'if constexpr' to avoid using
	pointer_traits::pointer_to when not necessary.
	(_List_base::_M_destroy_node): Fix parameter to be the pointer
	type used internally, not the allocator's pointer.
	(list::_M_create_node): Likewise.
	* testsuite/23_containers/forward_list/requirements/explicit_instantiation/alloc_ptr.cc:
	Check explicit instantiation of non-trivial value type.
	* testsuite/23_containers/list/requirements/explicit_instantiation/alloc_ptr.cc:
	Likewise.
	* testsuite/23_containers/forward_list/requirements/explicit_instantiation/alloc_ptr_ignored.cc:
	Fix macro name.
	* testsuite/23_containers/list/requirements/explicit_instantiation/alloc_ptr_ignored.cc:
	Likewise.

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