Fix seqfault in emission of NVPTX due to missing DIsubprogram

[vchuravy]

Fixes #576

@wsmoses now the only question is: Why is CI passing.

[wsmoses]

LGTM. I remember commenting this out earlier actually to fix a prior debug info issue (presumably the since fixed GPUCompiler one?)

[vchuravy]

So nested_codegen is breaking this:

                        cf = nested_codegen!(mod, add_one_in_place, Tuple{Any})
                        push!(function_attributes(cf), EnumAttribute("alwaysinline", 0; ctx))
                        permit_inlining!(cf)

[vchuravy]

I suspect it is due to the call not having a !dbg info.