Auto merge of #6771 - MortenLohne:master, r=flip1995

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`

clippy_lints/src/inherent_to_string.rs

@@ -106,6 +106,7 @@ impl<'tcx> LateLintPass<'tcx> for InherentToString {
             let decl = &signature.decl;
             if decl.implicit_self.has_implicit_self();
             if decl.inputs.len() == 1;
+            if impl_item.generics.params.is_empty();
 
             // Check if return type is String
             if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id), sym::string_type);