Avoid guessing unknown trait impl in suggestions

When a trait is used without specifying the implementation (e.g. calling
a non-member associated function without fully-qualified syntax) and
there are multiple implementations available, use a placeholder comment
for the implementation type in the suggestion instead of picking a
random implementation.

Example:

```
fn main() {
    let _ = Default::default();
}
```

Previous output:

```
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
 --> test.rs:2:13
  |
2 |     let _ = Default::default();
  |             ^^^^^^^^^^^^^^^^ cannot call associated function of trait
  |
help: use a fully-qualified path to a specific available implementation (273 found)
  |
2 |     let _ = <FileTimes as Default>::default();
  |             +++++++++++++        +
```

New output:

```
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
 --> test.rs:2:13
  |
2 |     let _ = Default::default();
  |             ^^^^^^^^^^^^^^^^ cannot call associated function of trait
  |
help: use a fully-qualified path to a specific available implementation (273 found)
  |
2 |     let _ = </* self type */ as Default>::default();
  |             +++++++++++++++++++        +
```

0 files changed, 0 insertions, 0 deletions