Create a new class: AttributeSetNode.

This is a helper class for the AttributeSetImpl class. It holds a set of
attributes that apply to a single element: function, return type, or
parameter.

These are uniqued.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173310 91177308-0d34-0410-b5e6-96231b3b80d8