libstdc++: Fix macro redefinition warnings

libstdc++: Fix macro redefinition warnings

Including <version> after <iterator> gives a warning about redefining
the __cpp_lib_array_constexpr macro. What happens is that <iterator>
sets the C++20 value, then <version> redefines it to the C++17 value,
then undefines it and defines it again to the C++20 value.

This change avoids defining it to the C++17 value when compiling C++20
or later (which also means we no longer need the #undef).

A similar warning happens for __cpp_lib_constexpr_char_traits when
including <version> after any header that includes <bits/char_traits.h>.

libstdc++-v3/ChangeLog:

	* include/std/version (__cpp_lib_array_constexpr):
	(__cpp_lib_constexpr_char_traits): Only define C++17 value when
	compiling C++17.