Convert getAttributes() to return an AttributeSetNode.

The AttributeSetNode contains all of the attributes. This removes one (hopefully
last) use of the Attribute class as a container of multiple attributes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173761 91177308-0d34-0410-b5e6-96231b3b80d8