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

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203857 91177308-0d34-0410-b5e6-96231b3b80d8