Don't display default generic parameters in diagnostics that compare types
In errors like:
```
expected type: `RawVec<foo, Global>`
found type: `foo`
```
`RawVec` being defined as `RawVec<T, A: Alloc = Global>`, the error is better written as
```
expected type: `RawVec<foo>`
found type: `foo`
```
In fact, that is already what happens when `foo` is not an ADT, because in that case, the diagnostic handler doesn't try to highlight something, and just uses the `Display` trait instead of its own logic.
e.g.
```
expected type: `RawVec<usize>`
found type: `usize`
```
0a8275f8b6