[C++11] Replacing ObjCInterfaceDecl iterators visible_extensions_begin() and visible_extensions_end() with iterator_range visible_extensions(). Updating all of the usages of the iterators with range-based for loops.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203855 91177308-0d34-0410-b5e6-96231b3b80d8