[OpenMP] Introduce the OpenMPOpt transformation pass

The OpenMPOpt pass is a CGSCC pass in which OpenMP specific
optimizations can reside.

The OpenMPOpt pass uses the OpenMPKinds.def file to identify runtime
calls and their uses. This allows targeted transformations and eases
their implementation.

This initial patch deduplicates `__kmpc_global_thread_num` and
`omp_get_thread_num` calls. We can also identify arguments that are
equivalent to such a call result and use it instead. Later we can
determine "gtid" arguments based on the use in kernel functions etc.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D69930