[NVPTX] Remove string constants from NVPTXBaseInfo.h.

Summary:
Previously they were defined as a 2D char array in a header file.  This
is kind of overkill -- we can let the linker lay out these strings
however it pleases.  While we're at it, we might as well just inline
these constants where they're used, as each of them is used only once.

Also move NVPTXUtilities.{h,cpp} into namespace llvm.

Reviewers: tra

Subscribers: jholewinski, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D27636

llvm-svn: 289728