[clang] [llvm] [X86] Extend kCFI with a 3-bit arity indicator (PR #121070)

[Scott Constable via llvm-commits]

scottconstable wrote:

> And you cannot use ESI when it's used to pass arguments.

@phoebewang I think that my https://github.com/llvm/llvm-project/pull/121070#discussion_r1898074522 also applies to this concern.

https://github.com/llvm/llvm-project/pull/121070