[DEBUG] Initial adaptation of NVPTX target for debug info emission.

Summary:
Patch adds initial emission of the debug info for NVPTX target.
Currently, only .file and .loc directives are emitted, everything else is
commented out to not break the compilation of Cuda.

Reviewers: echristo, jlebar, tra, jholewinski

Subscribers: mgorny, aprantl, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D41827

llvm-svn: 330271