[OpenMP] Extend CLANG target options with device offloading kind.

Summary: Pass the type of the device offloading when building the tool chain for a particular target architecture. This is required when supporting multiple tool chains that target a single device type. In our particular use case, the OpenMP and CUDA tool chains will use the same ```addClangTargetOptions ``` method. This enables the reuse of common options and ensures control over options only supported by a particular tool chain.

Reviewers: arpith-jacob, caomhin, carlo.bertolli, ABataev, jlebar, hfinkel, tstellar, Hahnfeld

Reviewed By: hfinkel

Subscribers: jgravelle-google, aheejin, rengolin, jfb, dschuff, sbc100, cfe-commits

Differential Revision: https://reviews.llvm.org/D29647

llvm-svn: 307272