[C++11] Replacing Decl iterators attr_begin() and attr_end() with iterator_range attrs(). Updating all of the usages of the iterators with range-based for loops.

This is a reapplication of r203236 with modifications to the definition of attrs() and following the new style guidelines on auto usage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203362 91177308-0d34-0410-b5e6-96231b3b80d8