The base template for std::char_traits<> has been removed in LLVM 19;
specializations are only provided for char, wchar_t, char8_t, char16_t,
and char32_t. Use appropriate type and constify the first argument
as requested by the compiler (char8_t is from C++20, but this matches
already requested CMAKE_CXX_STANDARD).
Nagged by: pkg-fallout
