Add constexpr to <iterator> and <array> for C++17

Add constexpr to <iterator> and <array> for C++17

	* include/bits/c++config (_GLIBCXX17_CONSTEXPR): Define.
	* include/bits/range_access.h (begin, end, rbegin, rend, crbegin)
	(crend): Add _GLIBCXX17_CONSTEXPR as per P0031R0.
	* include/bits/stl_iterator.h (reverse_iterator, move_iterator)
	(__make_reverse_iterator, make_reverse_iterator, make_move_iterator):
	Likewise.
	* include/bits/stl_iterator_base_funcs.h (__distance, __advance): Add
	_GLIBCXX14_CONSTEXPR.
	(distance, advance, next, prev): Add _GLIBCXX17_CONSTEXPR.
	* include/std/array (array::begin, array::end, array::rbegin)
	(array::rend, array::cbegin, array:cend, array::crbegin)
	(array::crend, array::operator[], array::at, array::front)
	(array::back, array::data): Likewise.
	* testsuite/24_iterators/headers/iterator/range_access.cc: Replace
	with separate tests for C++11, C++14, and C++17.
	* testsuite/24_iterators/headers/iterator/range_access_c++11.cc: New.
	* testsuite/24_iterators/headers/iterator/range_access_c++14.cc: New.
	* testsuite/24_iterators/headers/iterator/range_access_c++17.cc: New.

From-SVN: r239690