[CUDA] Add conversion operators for threadIdx, blockIdx, gridDim, and blockDim to uint3 and dim3.

Summary:
This lets you write, e.g.

  uint3 a = threadIdx;
  uint3 b = blockIdx;
  dim3 c = gridDim;
  dim3 d = blockDim;

which is legal in nvcc, but was not legal in clang.

The fact that e.g. the type of threadIdx is not actually uint3 is still
observable, but now you have to try to observe it.

Reviewers: tra

Subscribers: echristo, cfe-commits

Differential Revision: http://reviews.llvm.org/D17561

llvm-svn: 261777