[libc++] Make sure we implement and test LWG2280 properly (#67670)

We did not mark std::begin/std::end as noexcept for C-style arrays, we
did not have conditional noexcept on cbegin/cend, and we did not mark
array cbegin/cend as constexpr in all Standard modes. Since this is a
LWG issue, we should implement it as a DR in all Standard modes as
usual.

This patch fixes these issues and adds test coverage. Fixes #67471.