[OpenMP] Add flag for disabling thread state in runtime

The runtime uses thread state values to indicate when we use an ICV or
are in nested parallelism. This is done for OpenMP correctness, but it
not needed in the majority of cases. The new flag added is
`-fopenmp-assume-no-thread-state`.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D120106