[SE] Clean up device and host memory slices

Summary:
* Add LLVM_ATTRIBUTE_UNUSED_RESULT used to slicing methods in order to
  emphasize that the slicing is not done in place.
* Change device memory slice function name from `drop_front` to `slice`
  in order to match the naming convention of `llvm::ArrayRef` and host
  memory slice.
* Change the parameter names of host memory slice functions to
  `DropCount` and `TakeCount` to match device memory slice declarations.

Reviewers: jlebar

Subscribers: jprice, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24464

llvm-svn: 281239