Prefer addAttr(Attribute::AttrKind) over the AttributeList overload

This should simplify the call sites, which typically want to tweak one
attribute at a time. It should also avoid creating ephemeral
AttributeLists that live forever.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300718 91177308-0d34-0410-b5e6-96231b3b80d8