[OpenMP] Add option to assert no nested OpenMP parallelism on the GPU

The OpenMP device runtime needs to support the OpenMP standard. However
constructs like nested parallelism are very uncommon in real application
yet lead to complexity in the runtime that is sometimes difficult to
optimize out. As a stop-gap for performance we should supply an argument
that selectively disables this feature. This patch adds the
`-fopenmp-assume-no-nested-parallelism` argument which explicitly
disables the usee of nested parallelism in OpenMP.

Reviewed By: carlo.bertolli

Differential Revision: https://reviews.llvm.org/D132074