[clang] [llvm] [Clang][LFI] Add mlfi Clang option to control LFI subtarget features (PR #204689)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

> Was there an RFC for this?

LFI is a (nascent) backend tech; technically a subtly-different ABI from something like aarch64-linux-gnu.

https://llvm.org/docs/LFI.html

TL;DR it rewrites loads/stores/syscalls to create a "faster sandbox."

This PR adds controls to whether we skip sanitizing loads or stores individually.

> does GCC support this under this spelling

GCC does not support LFI at all (yet).

> is an -m flag the right design

I think so as most `-m` flags have implications on the ABI, but am open to suggestions.

https://github.com/llvm/llvm-project/pull/204689