rustdoc-json: Include GenericParamDefKind::Type::synthetic in JSON

rustdoc-json: Include GenericParamDefKind::Type::synthetic in JSON

The rustdoc JSON for

```
pub fn f(_: impl Clone) {}
```

will effectively be

```
pub fn f<impl Clone: Clone>(_: impl Clone)
```

where a synthetic generic parameter called `impl Clone` with generic
trait bound `Clone` is added to the function declaration.

The generated HTML filters out these generic parameters by doing
`self.params.iter().filter(|p| !p.is_synthetic_type_param())`, because
the synthetic generic parameter is not of interest to regular users.

For the same reason, we should expose whether or not a generic parameter
is synthetic or not also in the rustdoc JSON, so that rustdoc JSON
clients can also have the option to hide synthetic generic parameters.