[DFSan] Add flag to insert event callbacks.

Summary:
For now just insert the callback for stores, similar to how MSan tracks
origins.  In the future we may want to add callbacks for loads, memcpy,
function calls, CMPs, etc.

Reviewers: pcc, vitalybuka, kcc, eugenis

Reviewed By: vitalybuka, kcc, eugenis

Subscribers: eugenis, hiraditya, #sanitizers, llvm-commits, kcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D75312