Error out against additional non-auto trait bounds

[CohenArthur]

There needs to be special handling around bounds like these:

trait A {}
trait B {}
trait C {}

impl dyn A + B {}
impl dyn A + B + C {}

This code needs to error out because B and C are not auto traits

This depends on #3007 being solved properly.

[CohenArthur]

We can also take care of this later, as it's an error and we must first focus on handling correct code. I'll move this out of the Auto traits improvement milestone