[Attributor] Implement AANoAliasCallSiteArgument initialization

Summary: This patch adds an appropriate `initialize` method for `AANoAliasCallSiteArgument`.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66927

llvm-svn: 370456