PR libstdc++/81912 make std::__iterator_category constexpr

PR libstdc++/81912 make std::__iterator_category constexpr

	PR libstdc++/81912
	* include/bits/stl_iterator_base_types.h (__iterator_category): Add
	constexpr for C++11 and later.
	* testsuite/24_iterators/container_access.cc: Add target selector.
	* testsuite/24_iterators/range_access.cc: Fix clause number in
	comment.
	* testsuite/24_iterators/range_access_cpp14.cc: Likewise.
	* testsuite/24_iterators/range_access_cpp17.cc: New.

From-SVN: r251234