[DFSan] Add __dfsan_load_callback.

Summary:
When -dfsan-event-callbacks is specified, insert a call to
__dfsan_load_callback() on every load.

Reviewers: vitalybuka, pcc, kcc

Reviewed By: vitalybuka, kcc

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

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D75363