[Attributor] Create helper struct for handling analysis getters

Summary: This patch introduces a helper struct `AnalysisGetter` to put together analysis getters. In this patch, a getter for `AAResult` is also added for  `noalias`.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67603

llvm-svn: 372072