[clang] [llvm] [LFI][X86] Add X86 LFI target and system instruction rewrites (PR #189569)

[Zachary Yedidia via cfe-commits]

zyedidia wrote:

While the [x86 LFI RFC](https://discourse.llvm.org/t/rfc-lightweight-fault-isolation-lfi-target-for-x86-64/90774) is in discussion (particularly the bundling component), we'd like to continue developing the rewriter component to avoid being blocked on bundling. Please let me know if there are more changes to make to this PR before we can move forward.

https://github.com/llvm/llvm-project/pull/189569