[libc++] [LIBCXX-DEBUG-FIXME] Constexpr char_traits::copy mustn't compare unrelated pointers.

Now that __builtin_is_constant_evaluated() is present on all supported
compilers, we can use it to skip the UB-inducing assert in cases where
the computation might be happening at constexpr time.

Differential Revision: https://reviews.llvm.org/D101674