[libc++] Use saturation builtins directly for {add,sub}_sat (#165228)

This doesn't improve performance (except with optimizations disabled),
since the compiler is able to fold our current implementation. However,
it does significantly reduce the amount of code the compiler has to sift
through, reducing compile times a bit.

GitOrigin-RevId: d4222bf9e2175dc8d0707442802a222d652d0116
1 file changed