Fix FP in inherent_to_string when the function has generic parameters
Minimal example of the false positive:
````
struct G;
impl G {
fn to_string<const _N: usize>(&self) -> String {
"G.to_string()".to_string()
}
}
fn main() {
let g = G;
g.to_string::<1>();
}
````
Clippy emits an `inherent_to_string` warning, and suggests that we implement `Display` for `G` instead. However, this is not possible, since the generic parameter _N only exists in this function, not in `G` itself. This particular example uses const generics, which is where the issue is most likely to come up, but this PR skips the lint if the `to_string` function has any kind of generic parameters.
changelog: Fix FP in `inherent_to_string`
728f3976f0