Addition and subtraction operators std::array::iterator are defined only for
ptrdiff_t, which is signed, instead of size_t, which is unsigned. Therefore,
adding the index variable to ar.begin() will trigger -Wsign-conversion if
std::array::iterator is not a raw pointer because the index variable will be
implicitly converted from size_t (an unsigned type) to ptrdiff_t (a signed
type). To fix this, we explicitly static_cast index to a ptrdiff_t.
PiperOrigin-RevId: 613662928
Change-Id: I5e06c2261d7b8f167fae7bb6acece076257f8579