Fix unused variable warnings in -asserts build

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173996 91177308-0d34-0410-b5e6-96231b3b80d8
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