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.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@225242 138bc75d-0d04-0410-961f-82ee72b054a4