libstdc++: Simplify detection idiom using concepts

libstdc++: Simplify detection idiom using concepts

Add a simpler definition of std::__detected_or using concepts.  This
also replaces the __detector::value_t member which should have been using
a reserved name.

Use __detected_or in pointer_traits.

libstdc++-v3/ChangeLog:

	* include/bits/alloc_traits.h (allocator_traits::is_always_equal):
	Only instantiate is_empty if needed.
	* include/bits/ptr_traits.h (__ptr_traits_impl::difference_type)
	(__ptr_traits_impl::rebind): Use __detected_or.
	* include/experimental/type_traits (is_same_v): Add a partial
	specialization instead of instantiating the std::is_same class
	template.
	(detected_t): Redefine in terms of detected_or_t.
	(is_detected, is_detected_v): Redefine in terms of detected_t.
	* include/std/type_traits [__cpp_concepts] (__detected_or): Add
	new definition using concepts.
	(__detector::value_t): Rename to __is_detected.
	* testsuite/17_intro/names.cc: Check value_t isn't used.