[OpenMP] Fix sign comparison warnings from GCC

New affinity patch introduced legitimate sign-compare warnings that
clang doesn't report but GCC-10 does. This removes the warnings by
changing two variables types to unsigned.

Differential Revision: https://reviews.llvm.org/D95818

GitOrigin-RevId: ffca74b8b82f0af0d213bffa59c40b10449fd05f
1 file changed