[Driver][OpenMP] Add specialized action builder for OpenMP offloading actions.

Summary:
This patch adds a new specialized action builder to create OpenMP offloading actions. The specialized builder is added to the action builder already containing the CUDA specialized builder.

OpenMP offloading dependences between host and device actions (expressed with OffloadActions) are different that what is used for CUDA:
 - Device compile action depends on the host compile action - the device frontend extracts the information about the declarations that have to be emitted by looking into the metadata produced by the host frontend.
 - The host link action depends on the device link actions - the device images are embedded in the host binary at link time.

Reviewers: echristo, tra, rsmith, jlebar, ABataev, hfinkel

Subscribers: mkuron, whchung, mehdi_amini, cfe-commits, Hahnfeld, andreybokhanko, arpith-jacob, carlo.bertolli, caomhin

Differential Revision: https://reviews.llvm.org/D21845

llvm-svn: 285314