Unions that convert `()` to a reference don't work

[fee1-dead]

This came up at #420, see more details there. Basic repro (a copy of MaybeUninit):

// build-pass

#[spirv_std::spirv(fragment)]
pub fn main() {
    union A { x: (), y: &'static glam::Vec4 }
    let a = A { x: () };
}

[fee1-dead]

One hacky workaround would be:

    fn store(&mut self, val: Self::Value, ptr: Self::Value, _align: Align) -> Self::Value {
        if let SpirvType::Pointer { pointee } = self.lookup_type(val.ty) && let SpirvType::Void = self.lookup_type(pointee) { 
            return val;
        }