[IR] Rename the class templates for the case iterator and case handle to

not collide with the naming convention for template *arguments*. In at
least one case they actually collided and this confuses MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300038 91177308-0d34-0410-b5e6-96231b3b80d8