[OpenMP] Pass -fopenmp-is-device to preprocessing and machine specific code generation stages

Summary: The preprocessing and code generation and optimization stages of the compiler are also passed the "-fopenmp-is-device" flag. This is used to trigger machine specific preprocessing and code generation when performing device offloading to an NVIDIA GPU via OpenMP directives.

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

Reviewed By: Hahnfeld

Subscribers: Hahnfeld, rengolin

Differential Revision: https://reviews.llvm.org/D29645

llvm-svn: 306691