Rollup merge of #118551 - RalfJung:extern-types-bugs, r=compiler-errors

more targeted errors when extern types end up in places they should not

Cc rust-lang/rust#115709 -- this does not fix that bug but it makes the panics less obscure and makes it more clear that this is a deeper issue than just a little codegen oversight. (In rust-lang/rust#116115 we decided we'd stick to causing ICEs here for now, rather than nicer errors. We can't currently show any errors pre-mono and probably we don't want post-mono checks when this gets stabilized anyway.)