[ADT] Pass ArrayRef::slice size_t instead of unsigned.

Also fix slice wrappers drop_front and drop_back.
The unittests are pretty awkward, but do the job; alternatives
welcome!

..and yes, I do have ArrayRefs with more than 4 billion elements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271546 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed