[OpenMP][CUDA] Do not forward OpenMP flags for CUDA device actions.

Summary:
This patch prevents OpenMP flags from being forwarded to CUDA device commands. That was causing the CUDA frontend to attempt to emit OpenMP code which is not supported.

This fixes the bug reported in https://llvm.org/bugs/show_bug.cgi?id=28723.

Reviewers: hfinkel, carlo.bertolli, arpith-jacob, kkwli0, tra, ABataev

Subscribers: caomhin, cfe-commits

Differential Revision: https://reviews.llvm.org/D22895

llvm-svn: 276979