Handle const effects inherited from parent correctly in `type_certainty`
This fixes a (debug) ICE in `type_certainty` that happened in the [k256 crate]. (I'm sure you can also specifically construct an edge test case that will run into type_certainty false positives visible outside of debug builds from this bug)
<details>
<summary>Minimal ICE repro</summary>
```rs
use std::ops::Add;
Add::add(1_i32, 1).add(i32::MIN);
```
</details>
The subtraction here overflowed:
436675b477/clippy_utils/src/ty/type_certainty/mod.rs (L209)
... when we have something like `Add::add` where `add` fn has 0 generic params but the `host_effect_index` is `Some(2)` (inherited from the parent generics, the const trait `Add`), and we end up executing `0 - 1`.
(Even if the own generics weren't empty and we didn't overflow, this would still be wrong because it would assume that a trait method with 1 generic parameter didn't have any generics).
So, *only* exclude the "host" generic parameter if it's actually bound by the own generics
changelog: none
[k256 crate]: https://github.com/RustCrypto/elliptic-curves/tree/master/k256
0b441d5ac5