[SE] Add CUDA platform

Summary:
Basic CUDA platform implementation and cmake infrastructure to control
whether it's used. A few important TODOs will be handled in later
patches:

* Log some error messages that can't easily be returned as Errors.
* Cache modules and kernels to prevent reloading them if someone tries to
  reload a kernel that's already loaded.
* Tolerate shared memory arguments for kernel launches.

Reviewers: jlebar

Subscribers: beanz, mgorny, jprice, jlebar, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24538

llvm-svn: 281524