[Driver][OpenMP] Create tool chains for OpenMP offloading kind.

Summary: This patch adds new logic to create the necessary tool chains to support offloading for OpenMP. The OpenMP related options are checked and the tool chains created accordingly. Diagnostics are emitted in case the options are illegal or express unknown targets.

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

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

Differential Revision: https://reviews.llvm.org/D21843

llvm-svn: 285311