[DEBUGINFO, NVPTX]Emit last debugging directives.

Summary:
We may end up with not emitted debug directives at the end of the module
emission. Patch fixes this problem emitting those last directives the
end of the module emission.

Reviewers: echristo

Subscribers: jholewinski, llvm-commits

Differential Revision: https://reviews.llvm.org/D54320

llvm-svn: 348495