CUDA lets users share structs between the host and device, so for that
and other reasons, primitive types such as ptrdiff_t should be the same
on both sides of the compilation.
Our code to do this wasn't entirely successful. In particular, we did a
bunch of work during the NVPTXTargetInfo constructor, only to override
it in the NVPTX{32,64}TargetInfo constructors. It worked well enough on
Linux and Mac, but Windows is LLP64, which is different enough to break
it.
This patch removes the NVPTX{32,64}TargetInfo classes entirely and fixes
the bug described above.