Remove a use of AttributeWithIndex.

Remove a use of AttributeWithIndex.

We want to remove AttributeWithIndex because it provides a non-encapsulated view
of the AttributeSetImpl object. Instead, use accessor methods and iterators.

Eventually, this code can be simplified because the Attribute object will hold
only one attribute instead of multiple attributes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173641 91177308-0d34-0410-b5e6-96231b3b80d8