[DEBUGINFO, NVPTX] Emit correct debug information for local variables.
NVPTX target dos not use register-based frame information. Instead it
relies on the artificial local_depot that is used instead of the frame
and the data for variables must be emitted relatively to this
Reviewers: tra, jlebar, echristo
Subscribers: jholewinski, aprantl, JDevlieghere, llvm-commits
Differential Revision: https://reviews.llvm.org/D45963