Implement N4502, the C++ Detection Idiom.

Implement N4502, the C++ Detection Idiom.

	* doc/xml/manual/status_cxx2017.xml: Update status table.
	* include/experimental/type_traits (void_t, is_detected,
	is_detected_v, detected_t, detected_or, detected_or_t,
	is_detected_exact, is_detected_exact_v, is_detected_convertible,
	is_detected_convertible_v): Define.
	* include/std/type_traits (__detector, __detected_or, __detected_or_t,
	__detected_or_t_): Define.
	* testsuite/experimental/type_traits/detection.cc: New.

From-SVN: r225242