[libc++][test] Fixes constexpr char_traits. (#90981)

The issue in nasty_char_traits was discovered by @StephanTLavavej who
provided
the solution they use in MSVC STL. This solution is based on that
example.

The same issue affects the constexpr_char_traits which was discovered in
https://github.com/llvm/llvm-project/pull/88389. This uses the same fix.

Fixes: https://github.com/llvm/llvm-project/issues/74221