[libc++] Don't trigger unsigned conversion warnings in std::advance

The Standard documents the signature of std::advance as

    template <class Iter, class Distance>
    constexpr void advance(Iter& i, Distance n);

Furthermore, it does not appear to put any restriction on what the type
of Distance should be. While it is understood that it should usually
be std::iterator_traits::difference_type, I couldn't find any wording
that mandates that. Similarly, I couldn't find wording that forces the
distance to be a signed type.

This patch changes std::advance to accept any type in the second argument,
which appears to be what the Standard mandates. We then coerce it to the
iterator's difference type, but that's an implementation detail.

Differential Revision: https://reviews.llvm.org/D81425