libstdc++: Change __cpp_lib_array_constexpr for C++17 again

This partially reverts my previous change related to this macro. The
C++20 constexpr iterator requirements are always met by array:iterator,
because it's just a pointer. So the macro can be set to 201803 even in
C++17 mode.

	* include/bits/stl_iterator.h (__cpp_lib_array_constexpr): Revert
	value for C++17 to 201803L because P0858R0 is supported for C++17.
	* include/std/version (__cpp_lib_array_constexpr): Likewise.
	* testsuite/23_containers/array/element_access/constexpr_c++17.cc:
	Check for value corresponding to P0031R0 features being tested.
	* testsuite/23_containers/array/requirements/constexpr_iter.cc:
	Check for value corresponding to P0858R0 features being tested.