[Attributor] Create getName() method for abstract attribute

Summary: The `getName()` method returns the name of the abstract attribute

Reviewers: jdoerfert, sstefan1, uenoku, homerdin, baziotis

Reviewed By: sstefan1

Subscribers: uenoku, kuter, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83109