There is no `threadDim` in CUDA. Instead, it is `blockDim`. Then the current
`blockDim` is `gridDim` in CUDA.
Reviewed By: jhuber6
Differential Revision: https://reviews.llvm.org/D157051
The new ompx.h header will give us a place to put extensions. The first
are 3D getters for the common cuda values:
`{threadId,threadDim,blockId,blockDim}.{x,y,z}`
Differential Revision: https://reviews.llvm.org/D156501