Currently, in a situation like

extern {
    type Foo;
    static mut BAR: Foo;
}

we generate a zero-sized struct as the LLVM type for Foo. LLVM might use this to assume that BAR has size 0, which would be fatal. And anyway it seems prudent for extern types, that should match C's declared-but-not-defined types, to match what Clang does for the corresponding C types -- which is to use opaque.

See #58271 for an attempt to fix this, which ran into LLVM assertion failures.

Cc @eddyb