[DFSan] Add __dfsan_cmp_callback.

Summary:
When -dfsan-event-callbacks is specified, insert a call to
__dfsan_cmp_callback on every CMP instruction.

Reviewers: vitalybuka, pcc, kcc

Reviewed By: kcc

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

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D75389