Rollup merge of #41052 - topecongiro:overlapping_inherent_impls, r=estebank

Rollup merge of #41052 - topecongiro:overlapping_inherent_impls, r=estebank

Make 'overlapping_inherent_impls' lint a hard error

This is ought to be implemented in PR #40728. Unfortunately, when I rebased the PR to resolve merge conflict, the "hard error" code disappeared. This PR complements the initial PR.

Now the following rust code gives the following error:
```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}

fn main() {}
```
```
error[E0592]: duplicate definitions with name `id`
 --> /home/topecongiro/test.rs:4:5
  |
4 |     fn id() {}
  |     ^^^^^^^^^^ duplicate definitions for `id`
...
8 |     fn id() {}
  |     ---------- other definition for `id`

error: aborting due to previous error
```