Make sure that the Attribute object represents one attribute only.

Several places were still treating the Attribute object as respresenting
multiple attributes. Those places now use the AttributeSet to represent
multiple attributes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174004 91177308-0d34-0410-b5e6-96231b3b80d8