Collect all unreachable fields in a single struct literal struct and emit a single error, instead of one error per private field.
```
error[E0451]: fields `beta` and `gamma` of struct `Alpha` are private
--> $DIR/visibility.rs:18:13
|
LL | let _x = Alpha {
| ----- in this type
LL | beta: 0,
| ^^^^^^^ private field
LL | ..
| ^^ field `gamma` is private
```
```
error[E0451]: field `beta` of struct `Alpha` is private
--> $DIR/visibility.rs:11:37
|
LL | let x = crate::foo::Alpha { .. };
| ^^ field `beta` is private
```