[CUDA] Updated CompileCudaWithLLVM doc.

Prerequisites

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CUDA is supported since llvm 3.9. Current release of clang (7.0.0) supports CUDA

7.0 through 9.2. If you need support for CUDA 10, you will need to use clang

built from r342924 or newer.

CUDA is supported since llvm 3.9. Clang currently supports CUDA 7.0 through

10.1. If clang detects a newer CUDA version, it will issue a warning and will

attempt to use detected CUDA SDK it as if it were CUDA-10.1.

Before you build CUDA code, you'll need to have installed the appropriate driver

for your nvidia GPU and the CUDA SDK.  See `NVIDIA's CUDA installation guide

Before you build CUDA code, you'll need to have installed the CUDA SDK.  See

`NVIDIA's CUDA installation guide

<https://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html>`_ for

details.  Note that clang `does not support

details.  Note that clang `maynot support

<https://llvm.org/bugs/show_bug.cgi?id=26966>`_ the CUDA toolkit as installed by

many Linux package managers; you probably need to install CUDA in a single

directory from NVIDIA's package.

some Linux package managers. Clang does attempt to deal with specific details of

CUDA installation on a handful of common Linux distributions, but in general the

most reliable way to make it work is to install CUDA in a single directory from

NVIDIA's `.run` package and specify its location via `--cuda-path=...` argument.

CUDA compilation is supported on Linux. Compilation on MacOS and Windows may or

may not work and currently have no maintainers. Compilation with CUDA-9.x is

`currently broken on Windows <https://bugs.llvm.org/show_bug.cgi?id=38811>`_.

may not work and currently have no maintainers.

Invoking clang

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