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

[ereslibre]

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 #45603 as stated in #42106 (comment).

This program, for example:

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As an aside, an outdated comment has been removed (Handler::cancel will only call to the Diagnostic::cancel method and will not decrease the count of errors).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

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @eddyb (or someone else) soon.

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[estebank]

@bors r+

[bors]

📌 Commit 063607e has been approved by estebank

[bors]

⌛ Testing commit 063607e with merge 0f4ebf9...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: estebank
Pushing 0f4ebf9 to master...