[CUDA] Fix libdevice selection.

This makes clang's libdevice selection match that of NVCC as described in
http://docs.nvidia.com/cuda/libdevice-users-guide/basic-usage.html#version-selection

If required libdevice variant is not found, driver now fails with an error.

Differential Revision: https://reviews.llvm.org/D23037

llvm-svn: 277542