[C++11] Replacing DeclContext iterators using_directives_begin() and using_directives_end() with iterator_range using_directives(). Updating all of the usages of the iterators with range-based for loops, and removing the no-longer-needed iterator versions. Also used as an opportunity to normalize the name from getUsingDirectives() to using_directives().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204061 91177308-0d34-0410-b5e6-96231b3b80d8