Miscompilation of a program projecting field of an extern type

[tmiasko]

Since #118534 projecting a field of an extern type at non-zero offset panics. MIR transformations do not understand this:

$ cat a.rs
#![feature(extern_types)]
extern "C" {
    type Opaque;
}
fn main() {
    let a = [0u8; 1];
    let b: &(u8, Opaque) = unsafe { &*(&a as *const _ as *const (u8, Opaque)) };
    let _ = &b.1;
}
$ rustc a.rs -Zmir-opt-level=1 && ./a
$ rustc a.rs -Zmir-opt-level=0 && ./a
thread 'main' panicked at library/core/src/panicking.rs:221:5:
attempted to compute the size or alignment of extern type `Opaque`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.
Aborted