Remove the last of uses that use the Attribute object as a collection of attributes.

Collections of attributes are handled via the AttributeSet class now. This
finally frees us up to make significant changes to how attributes are structured.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173229 91177308-0d34-0410-b5e6-96231b3b80d8