ptr_traits.h (__ptrtr_elt_type, [...]): Remove

ptr_traits.h (__ptrtr_elt_type, [...]): Remove

	* include/bits/ptr_traits.h (__ptrtr_elt_type, __ptrtr_diff_type,
	__ptrtr_rebind, __ptrtr_not_void): Remove
	(__get_first_arg, __replace_first_arg, __make_not_void): Define new
	transformations.
	(__detected_or_): New detection trait.
	(pointer_traits): Use new traits.
	* testsuite/20_util/pointer_traits/pointer_to.cc: Add rebind member.
	* testsuite/20_util/pointer_traits/requirements/
	explicit_instantiation.cc: Use valid arguments to pointer_traits.

From-SVN: r225243