Auto merge of #47146 - ereslibre:issue-42106, r=estebank

Auto merge of #47146 - ereslibre:issue-42106, r=estebank

Only bump error count when we are sure that the diagnostic is not a repetition

This ensures that if we emit the same diagnostic twice, the error count will
match the real number of errors shown to the user.

Fixes #42106

This is a followup of https://github.com/rust-lang/rust/pull/45603 as stated in https://github.com/rust-lang/rust/issues/42106#issuecomment-345218473.

This program, for example:

```rust
fn do_something<T>(collection: &mut Vec<T>) {
    let _a = &collection;
    collection.swap(1, 2);
}

fn main() {}
```

without this patch, produces:

```
error[E0502]: cannot borrow `*collection` as mutable because `collection` is also borrowed as immutable
  --> $DIR/issue-42106.rs:13:5
   |
12 |     let _a = &collection;
   |               ---------- immutable borrow occurs here
13 |     collection.swap(1, 2); //~ ERROR also borrowed as immutable
   |     ^^^^^^^^^^ mutable borrow occurs here
14 | }
   | - immutable borrow ends here

error: aborting due to 2 previous errors
```

The number of errors do not match the diagnostics reported. This PR fixes this problem. The output is now in this case:

```
error[E0502]: cannot borrow `*collection` as mutable because `collection` is also borrowed as immutable
  --> $DIR/issue-42106.rs:13:5
   |
12 |     let _a = &collection;
   |               ---------- immutable borrow occurs here
13 |     collection.swap(1, 2); //~ ERROR also borrowed as immutable
   |     ^^^^^^^^^^ mutable borrow occurs here
14 | }
   | - immutable borrow ends here

error: aborting due to previous error
```

Also, some other tests outputs have been adapted because their count didn't really match the number of diagnostics reported.

As an aside, an outdated comment has been removed (`Handler::cancel` will only call to the `Diagnostic::cancel` method and will not decrease the count of errors).

All tests are passing with this PR (`x.py test` is successful).