[OpenMP] Don't throw cudalib not found error if only front-end is required.

Summary: If we only use the compiler front-end, do not throw an error about the cuda device library not being found. This allows the front-end to be run on systems where no Cuda installation is found.

Reviewers: Hahnfeld, ABataev, carlo.bertolli, caomhin, tra

Reviewed By: tra

Subscribers: hfinkel, tra, cfe-commits

Differential Revision: https://reviews.llvm.org/D37914

llvm-svn: 314150