Use the AttributeSet instead of AttributeWithIndex.

In the future, AttributeWithIndex won't be used anymore. Besides, it exposes the
internals of the AttributeSet to outside users, which isn't goodness.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173605 91177308-0d34-0410-b5e6-96231b3b80d8