ADT: Add a drop_back() helper to ArrayRef

The slice(N, M) interface is powerful but not concise when wanting to
drop a few elements off of an ArrayRef, fix this by adding a drop_back
method.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212370 91177308-0d34-0410-b5e6-96231b3b80d8