[CUDA] Use atexit() to call module destructor.

This matches the way NVCC does it. Doing module cleanup at global
destructor phase used to work, but is, apparently, too late for
the CUDA runtime in CUDA-9.2, which ends up crashing with double-free.

Differential Revision: https://reviews.llvm.org/D48613

llvm-svn: 335763