[NVPTX] Initial adaptation of MCAsmStreamer/MCTargetStreamer for debug info in Cuda.

Summary:
Initial changes in interfaces of MCAsmStreamer/MCTargetStreamer for
correct debug info emission for Cuda.
1. PTX foramt does not support `.ascii` directives. Added the ability to
nullify it.
2. The initial function label must follow the first debug `.loc`
directive, not be followed by.
3. DWARF sections must be enclosed in braces.

Reviewers: hfinkel, probinson, jlebar, rafael, echristo

Subscribers: sdardis, nemanjai, llvm-commits, aprantl

Differential Revision: https://reviews.llvm.org/D40033

llvm-svn: 321178