abi_unsupported_vector_types lint is optimization-dependent

[RalfJung]

The lint guarding against #116558 is intended to eventually become a hard error to fix this ABI issue. However, right now the lint is optimization-dependent: it runs during monomorphization to be able to figure out the actual argument types that will be passed at all call sites, and reject them if they need a missing target feature. If a call gets optimized away (e.g. in dead code), the lint will not fire.

I don't know any good way to prevent this, since we need the monomorphization-time information to check whether any of the arguments is a too-large SIMD vector. So this issue mostly serves to let @rust-lang/lang know that this is a thing, and as a place to track any ideas we might have about fixing this, or concerns about making this into an optimization-dependent hard error.