The `rust-call` ABI isn't compatible with
`#![feature(unsized_fn_params)]`, so trying to use that feature with
closures leads to an ICE (#67981). This turns that ICE into a
type-check error.
Align unsized locals
Allocate an extra space for unsized locals and manually align the storage, since alloca doesn't support dynamic alignment.
Fixes#71416.
Fixes#71695.