Improve error message for missing trait in trait impl
Fixes#88818. For the following example:
```rust
struct S { }
impl for S { }
```
the current output is:
```
error: missing trait in a trait impl
--> t1.rs:2:5
|
2 | impl for S { }
| ^
```
With my changes, I get:
```
error: missing trait in a trait impl
--> t1.rs:2:5
|
2 | impl for S { }
| ^
|
help: add a trait here
|
2 | impl Trait for S { }
| +++++
help: for an inherent impl, drop this `for`
|
2 - impl for S { }
2 + impl S { }
|
```
84d65fee0e