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();
  |             +++++++++++++++++++        +
```

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -2382,17 +2382,21 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             && let Some(impl_def_id) = trait_impls.non_blanket_impls().values().flatten().next()
                         {
                             let non_blanket_impl_count = trait_impls.non_blanket_impls().values().flatten().count();
-                            let message = if non_blanket_impl_count == 1 {
+                            // If there is only one implementation of the trait, suggest using it.
-                                "use the fully-qualified path to the only available implementation".to_string()
+                            // Otherwise, use a placeholder comment for the implementation.
-                            } else {
+                            let (message, impl_suggestion) = if non_blanket_impl_count == 1 {(
+                                "use the fully-qualified path to the only available implementation".to_string(),
+                                format!("<{} as ", self.tcx.type_of(impl_def_id).subst_identity())
+                            )} else {(
                                 format!(
                                     "use a fully-qualified path to a specific available implementation ({} found)",
                                     non_blanket_impl_count
-                                )
+                                ),
-                            };
+                                "</* self type */ as ".to_string()
+                            )};
                             let mut suggestions = vec![(
                                 path.span.shrink_to_lo(),
-                                format!("<{} as ", self.tcx.type_of(impl_def_id).subst_identity())
+                                impl_suggestion
                             )];
                             if let Some(generic_arg) = trait_path_segment.args {
                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);

tests/ui/error-codes/E0283.stderr

@@ -9,8 +9,8 @@ LL |     let cont: u32 = Generator::create();
    |
 help: use a fully-qualified path to a specific available implementation (2 found)
    |
-LL |     let cont: u32 = <Impl as Generator>::create();
+LL |     let cont: u32 = </* self type */ as Generator>::create();
-   |                     ++++++++          +
+   |                     +++++++++++++++++++          +
 
 error[E0283]: type annotations needed
   --> $DIR/E0283.rs:35:24

tests/ui/error-codes/E0790.stderr

@@ -65,8 +65,8 @@ LL |     MyTrait2::my_fn();
    |
 help: use a fully-qualified path to a specific available implementation (2 found)
    |
-LL |     <Impl1 as MyTrait2>::my_fn();
+LL |     </* self type */ as MyTrait2>::my_fn();
-   |     +++++++++         +
+   |     +++++++++++++++++++         +
 
 error: aborting due to 5 previous errors