[C++11] Replacing ObjCCategoryDecl iterators ivar_begin() and ivar_end() with iterator_range ivars(). Updating all of the usages of the iterators with range-based for loops.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203924 91177308-0d34-0410-b5e6-96231b3b80d8