[DEBUGINFO, NVPTX] Disable emission of ',debug' option if only debug directives are allowed.

Summary:
If the output of debug directives only is requested, we should drop
emission of ',debug' option from the target directive. Required for
supporting of nvprof profiler.

Reviewers: echristo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46061

llvm-svn: 348497