[CUDA] Raise an error if the CUDA install can't be found.

Summary:
Without this change, we silently proceed on without including
__clang_cuda_runtime_wrapper.h.  This leads to very strange behavior --
you say you're compiling CUDA code, but e.g. __device__ is not defined!

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19180

llvm-svn: 266496