[libc++] Allow __move_constexpr to work with unrelated pointers

Allow `__move_constexpr` to work with unrelated pointers and `_LIBCPP_ASSERT` that `__copy_constexpr`, `__move_constexpr` and `__assign_constexpr` are only run during constant evaluation

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D115986