[C++11] Replacing UsingDecl iterators shadow_begin() and shadow_end() with iterator_range shadows(). Updating all of the usages of the iterators with range-based for loops.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203825 91177308-0d34-0410-b5e6-96231b3b80d8