In `ranges::advance(iter, n, bound)`, we'd incorrectly handle the case
where bound < iter and n is 0:
int a[10];
int *p = a+5;
int *bound = a+3;
std::ranges::advance(p, 0, bound);
assert(p - a == 5); // we'd return 3 before this patch
This was caused by an incorrect handling of 0 inside __magnitude_geq.
Differential Revision: https://reviews.llvm.org/D117240
d27cbfa9d3