Auto merge of #45603 - joshleeb:iss42106, r=estebank

Fix duplicate display of error E0502

Ref. Repeated "mutable/immutable borrow" error messages #42106.

This PR modifies the return type of [`report_error_if_loan_conflicts_with_restriction`](https://github.com/rust-lang/rust/blob/0f0f5db465de96b6c12e71f0c7d3e475f618b104/src/librustc_borrowck/borrowck/check_loans.rs#L398-L403) so the result can be checked in [`report_error_if_loans_conflict`](https://github.com/rust-lang/rust/blob/0f0f5db465de96b6c12e71f0c7d3e475f618b104/src/librustc_borrowck/borrowck/check_loans.rs#L377-L396). This is done to prevent displaying a duplicate of the error message E0502 which is referenced in #42106.

The output of compiling:

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

fn main() {}
```

is now

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

error: aborting due to 2 previous errors
```

r? @estebank

0 files changed, 0 insertions, 0 deletions