benchmarking v2 syntax does not issue warnings on unused benchmark params

[sam0x17]

Because of how the benchmark function defs aren't actually real function defs, we don't get nice warnings about unused benchmark parameters that we would get if these were real functions.

ie this should issue a warning about x not being used, but does not:

#[benchmark]
fn my_benchmark(x: Linear<10, 100>) {
    #[block]
    {}
}

This would be fixed if we generate real function definitions, as contemplated in #13224