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.
* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc:
Likewise.
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