[CUDA] Add hack so code which includes "curand.h" doesn't break.

Summary:
curand.h includes curand_mtgp32_kernel.h.  In host mode, this header
redefines threadIdx and blockDim, giving them their "proper" types of
uint3 and dim3, respectively.

clang has its own plan for these variables -- their types are magic
builtin classes.  So these redefinitions are incompatible.

As a hack, we force-include the offending CUDA header and use #defines
to get the right types for threadIdx and blockDim.

Reviewers: tra

Subscribers: echristo, cfe-commits

Differential Revision: http://reviews.llvm.org/D17562

llvm-svn: 261776