Cbc ignores parameters when model is an LP

Cbc ignores the outcome of `set_silent` when the model is an LP. I guess this is because the attribute isn't passed to CLP.

MWE:

```julia
using JuMP, Cbc
Cbc_optimizer = JuMP.optimizer_with_attributes(Cbc.Optimizer)
m = Model(Cbc_optimizer)
x = @variable(m, upper_bound = 0)
y = @variable(m, upper_bound = 0)
@objective(m, Min, x + y)
optimize!(m)
```