Define __void_t and SFINAE-friendly iterator_traits.

Define __void_t and SFINAE-friendly iterator_traits.

	* include/std/type_traits (__void_t): Define new alias template.
	(_GLIBCXX_HAS_NESTED_TYPE): Redefine using __void_t.
	* include/std/functional (_Maybe_get_result_type): Likewise.
	* include/bits/stl_iterator_base_types.h (__iterator_traits): Likewise.
	* include/bits/uses_allocator.h (__uses_allocator_helper): Likewise.
	* testsuite/20_util/bind/ref_neg.cc: Adjust dg-error.
	* testsuite/20_util/reference_wrapper/typedefs-3.cc: Adjust to changes
	in _GLIBCXX_HAS_NESTED_TYPE.

From-SVN: r217395