I removed the copy ctor, thinking that'd be the end of it - these
iterators should be perfectly assignable even from disjoint ranges (as
any iterator would be) - exkcept that the member was const.
Unconstify it.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231146 91177308-0d34-0410-b5e6-96231b3b80d8