[doc] Compile CUDA with LLVM

Summary:
This patch adds documentation on compiling CUDA with LLVM as requested by many
engineers and researchers. It includes not only user guides but also some
internals (mostly optimizations) so that early adopters can start hacking and
contributing.

Quite a few researchers who contacted us haven't used LLVM before, which is
unsurprising as it hasn't been long since LLVM picked up CUDA. So I added a
short summary to help these folks get started with LLVM.

I expect this document to evolve substantially down the road. The user guides
will be much simplified after the Clang integration is done. However, the
internals should continue growing to include for example performance debugging
and key areas to improve.

Reviewers: chandlerc, meheff, broune, tra

Subscribers: silvas, jingyue, llvm-commits, eliben

Differential Revision: http://reviews.llvm.org/D14370

llvm-svn: 252660