[OpenMP] Make bc file compilation sensitive to LIBOMPTARGET_NVPTX_DEBUG flag

Summary: The LIBOMPTARGET_NVPTX_DEBUG flag is inconsistent between using nvcc to generate .a file and clang to generate .bc file. Sync the two setting so we can get debug messages from the bc file path as well.

Reviewers: grokos

Subscribers: Hahnfeld, openmp-commits, mgorny

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D45530

llvm-svn: 330477