Rewrite the addAttr() method.

This now uses the AttributeSet object instead of the Attribute /
AttributeWithIndex objects. It's fairly simple now. It goes through all of the
subsets before the one we're modifying, adds them to the new set. It then adds
the modified subset. And then adds the rest of the subsets.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173659 91177308-0d34-0410-b5e6-96231b3b80d8