libstdc++: Update value of __cpp_lib_constexpr_char_traits for C++20

libstdc++: Update value of __cpp_lib_constexpr_char_traits for C++20

Although not required by SD-6 or the C++20 draft, we define the macro
__cpp_lib_constexpr_char_traits to indicate support for P0432R1. This
updates the value in C++20 mode for the P1032R1 changes to char_traits.

	* include/bits/char_traits.h (__cpp_lib_constexpr_char_traits):
	Update value for C++20.
	* include/std/version (__cpp_lib_constexpr_char_traits): Likewise.
	* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc:
	Update expected value.

(cherry picked from commit b6ab9ecd550227684643b41e9e33a4d3466724d8)