We have it for StringRef but not ArrayRef, and ArrayRef has drop_back,
so I see no reason it shouldn't have drop_front. Splitting this out of a
change that I have that will use this funcitonality.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268434 91177308-0d34-0410-b5e6-96231b3b80d8