[clang] [llvm] [X86] Enhance kCFI type IDs with a 3-bit arity indicator. (PR #117121)

[Ramon de C Valle via cfe-commits]

rcvalle wrote:

> Flag guarding this feature seems like it would also be good for any existing C users - for example, if trying to build a kernel module intended to load against a kernel image built with an older `clang`, you need to select the same type ID projection that the kernel did.

+1 to this. Adding support for this to the Rust compiler shouldn't be a problem and @maurer or I could take a look at it. However, I wonder if the arity information should be from the high level information (i.e., from the function declarations/definitions) instead of from the lower level calling convention used/code generated (and possibly also affected by/after optimizations). This would ensure compatibility with any calling convention and any other possible differences that may come up later when using cross-language KCFI. (The KCFI encoding is based on high level type information from the function declarations/definitions.)

https://github.com/llvm/llvm-project/pull/117121