[ADT] Provide some helpful static_asserts for using operations of the
wrong iterator category. These aren't comprehensive, but they have
caught the common cases for me and produce much nicer errors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207601 91177308-0d34-0410-b5e6-96231b3b80d8