[libc++] Remove is_signed<T> use from <limits> (#168334)

`numeric_limits` already has an `is_signed` member. We can use that
instead of using `std::is_signed`.

NOKEYCHECK=True
GitOrigin-RevId: 8bfd29486e042af5b4e4a6e9ef5e6d04ce618651
13 files changed