[Attributor] Create getter function for the ID of the abstract attribute

Summary: The `getIdAddr()` function returns the address of the ID of the abstract attribute

Reviewers: jdoerfert, sstefan1, uenoku, homerdin, baziotis

Reviewed By: jdoerfert

Subscribers: okura, hiraditya, uenoku, kuter, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83172