PR libstdc++/72792
	* include/bits/alloc_traits.h (__allocator_traits_base::__diff_type)
	(__allocator_traits_base::__size_type): Remove.
	(allocator_traits::_Ptr): New class template to detect const and void
	pointer types without instantiating pointer_traits::rebind
	unnecessarily.
	(allocator_traits::_Diff): Likewise for detecting difference_type.
	(allocator_traits::_Size): New class template to detect size_type
	without instantiating make_unsigned unnecessarily.
	* include/bits/ptr_traits.h (pointer_traits::element_type): Use
	__detected_or_t instead of __detected_or_t_.
	* include/std/type_traits (__detected_or_t_): Remove.
	* testsuite/20_util/allocator_traits/members/pointers.cc: New test.

From-SVN: r244706