Disallow passing `_Complex` by value

[SabrinaJewson]

As seen in rust-lang/rfcs#793 and rust-lang/libc#355, C’s _Complex has a different ABI (but not layout) to what Bindgen currently generates, which is:

#[derive(PartialEq, Copy, Clone, Hash, Debug, Default)]
#[repr(C)]
pub struct __BindgenComplex<T> {
    pub re: T,
    pub im: T,
}

As a soundness fix, I suggest erroring when the user attempts to use _Complex by value. Maybe this could be done with a flag to disable it if the user really wants the potentially-unsound behaviour?