[DEBUG_INFO, NVPTX] Do not emit .debug_loc section.

Summary:
.debug_loc section is not supported for NVPTX target. If there is an
object whose location can change during its lifetime, we do not generate
debug location info for this variable.

Reviewers: echristo

Subscribers: jholewinski, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D48730

llvm-svn: 335976