[OpenMP] Use assumptions during ICV tracking

The OpenMP 5.1 assumptions `no_openmp` and `no_openmp_routines` allow us
to ignore calls that would otherwise prevent ICV tracking.

Once we track more ICVs we might need to distinguish the ones that could
be impacted even with `no_openmp_routines`.

Reviewed By: sstefan1

Differential Revision: https://reviews.llvm.org/D92050