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

[Zachary Yedidia via cfe-commits]

zyedidia wrote:

Thanks! We essentially want a frontend mechanism to control the backend subtarget features for the LFI subarchitecture. At the moment the only subtarget features are `+no-lfi-stores` and `+no-lfi-loads`, but thinking long-term there could more, such as a subtarget feature for sandbox size (4G vs. configurable), or segmentation-based masking specifically for the `x86_64_lfi` subarchitecture.

It looks like other subarchitectures like `arm64ec` don't have configurations knobs similar to what we want, but there are some security options like `mbranch-protection` and `mharden-sls` that are somewhat similar to the proposed `mlfi` option. Definitely open to suggestions though -- there was some small discussion in the original LFI [RFC](https://discourse.llvm.org/t/rfc-lightweight-fault-isolation-lfi-efficient-native-code-sandboxing-upstream-lfi-target-and-compiler-changes/88380) about Clang options, but discussion mostly focused on backend support.

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