[C++11] Replacing Decl iterators attr_begin() and attr_end() with...

[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.

llvm-svn: 203362