Auto merge of #122456 - maurer:cfi-nonpassed, r=workingjubilee

CFI: Skip non-passed arguments

Rust will occasionally rely on fn((), X) -> Y being compatible with fn(X) -> Y, since () is a non-passed argument. Relax CFI by choosing not to encode non-passed arguments.

This PR was split off from #121962 as part of fixing the larger vtable compatibility issues.

r? `@workingjubilee`

Author: bors