[CUDA][OPENMP][NVPTX]Improve logic of the debug info support.

Summary:
Added support for the -gline-directives-only option + fixed logic of the
debug info for CUDA devices. If optimization level is O0, then options
--[no-]cuda-noopt-device-debug do not affect the debug info level. If
the optimization level is >O0, debug info options are used +
--no-cuda-noopt-device-debug is used or no --cuda-noopt-device-debug is
used, the optimization level for the device code is kept and the
emission of the debug directives is used.
If the opt level is > O0, debug info is requested +
--cuda-noopt-device-debug option is used, the optimization is disabled
for the device code + required debug info is emitted.

Reviewers: tra, echristo

Subscribers: aprantl, guansong, JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D51554

llvm-svn: 348930