The flag will be used for the arm64e work we plan to upstream in the future (see https://lists.llvm.org/pipermail/llvm-dev/2019-October/136091.html). Passing KnownNonNull_t::True allows clang to avoid emitting null checks on arm64e. Currently the flag has no effect on code generation on other targets.
This is the first patch and there are many more places where KnownNonNull_t::True can be passed. I plan to fix those places in future patches.