[C++11] Replacing DeclContext iterators using_directives_begin() and...

[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().

llvm-svn: 204061