[NVPTX] Emit debug info in DWARF-2 by default for Cuda devices.

Summary:
NVPTX target supports debug info in DWARF-2 format. Patch adds emission
of debug info in DWARF-2 by default.

Reviewers: tra, jlebar

Subscribers: aprantl, JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D42581

llvm-svn: 330272