[dfsan] Add "DataFlow" option to LLVM_USE_SANITIZER

Summary:
This patch add the dataflow option to LLVM_USE_SANITIZER and documents
it.

Tested via check-cxx (wip to fix the errors).

Reviewers: morehouse, #libc!

Subscribers: mgorny, cfe-commits, libcxx-commits

Tags: #clang, #libc

Differential Revision: https://reviews.llvm.org/D78390