This patch adds the basic infrastructure for lowering an OpenMP
directive, which should enable someone to take over the OpenMP lowering
in the future. It adds the lowering entry points to CIR in the same way
as OpenACC.
Note that this does nothing with any of the directives, which will
happen in a followup patch. No infrastructure for clauses is added
either, but that will come in a followup patch as well.
