[CUDA] More correctly inherit primitive types from the host during device compilation.

Summary:
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.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28322

llvm-svn: 291135