[OpenMP] Add support for auxiliary triple specification

Summary: Device offloading requires the specification of an additional flag containing the triple of the //other// architecture the code is being compiled on if such an architecture exists. If compiling for the host, the auxiliary triple flag will contain the triple describing the device and vice versa.

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

Reviewed By: Hahnfeld

Subscribers: rengolin, cfe-commits

Differential Revision: https://reviews.llvm.org/D29339

llvm-svn: 306689