libstdc++: Update (and revert) value of __cpp_lib_array_constexpr

libstdc++: Update (and revert) value of __cpp_lib_array_constexpr

This macro should have been updated to 201811 when the last C++20
changes were implemented. However those changes are not enabled for
C++17 mode, so the macro should only have the new value in C++20 mode.

This change ensures that the macro is defined to 201603 for C++17 and
201811 for C++20.

	* include/bits/stl_iterator.h (__cpp_lib_array_constexpr): Define
	different values for C++17 and C++20, to indicate different feature
	sets. Update value for C++20 to indicate P1032R1 support.
	* include/std/version (__cpp_lib_array_constexpr): Likewise.
	* testsuite/23_containers/array/comparison_operators/constexpr.cc:
	Check feature test macro.
	* testsuite/23_containers/array/element_access/constexpr_c++17.cc:
	New test.
	* testsuite/23_containers/array/requirements/constexpr_fill.cc: Check
	feature test macro.
	* testsuite/23_containers/array/requirements/constexpr_iter.cc: Test
	in C++17 mode and check feature test macro.