[CUDA] Add utility functions for dealing with CUDA versions / architectures.

[CUDA] Add utility functions for dealing with CUDA versions / architectures.

Summary:
Currently our handling of CUDA architectures is scattered all around
clang.  This patch centralizes it.

A key advantage of this centralization is that you can now write a C++
switch on e.g. CudaArch and get a compile error if you don't handle one
of the enum values.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21867

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274681 91177308-0d34-0410-b5e6-96231b3b80d8