Limit instantiation of Vec and Box shims to local element type

[dtolnay]

This fixes a hole in the combination of #541 + #542 (which is why I'd excluded those PRs from the most recent release).

Without this restriction, the following crates would have typechecked but caused duplicate symbols which is UB:

// crate A

#[cxx::bridge]
mod ffi {
    struct S { x: i32 }
}

// crate B which depends on A

#[cxx::bridge]
mod ffi {
    extern "C++" {
        type S = a::S;
    }
    impl Vec<S> {}
}

// crate C which depends on A

#[cxx::bridge]
mod ffi {
    extern "C++" {
        type S = a::S;
    }
    impl Vec<S> {}
}

The nonlocal types design described in #496 (comment) prohibits the code in crates B and C (unsafe impl Vec<S> would be required for a nonlocal type S) but is not implemented yet. For now this PR is a pretty blunt restriction and will get substantially relaxed as #496 gets implemented.