Rollup merge of #88639 - Emilgardis:fix-issue-88600, r=GuillaumeGomez

Rollup merge of #88639 - Emilgardis:fix-issue-88600, r=GuillaumeGomez

rustdoc: Fix ICE with `doc(hidden)` on tuple variant fields

Fixes #88600.

```rust
pub struct H;
pub struct S;

pub enum FooEnum {
    HiddenTupleItem(#[doc(hidden)] H),
    MultipleHidden(#[doc(hidden)] H, #[doc(hidden)] H),
    MixedHiddenFirst(#[doc(hidden)] H, S),
    MixedHiddenLast(S, #[doc(hidden)] H),
    HiddenStruct {
        #[doc(hidden)]
        h: H,
        s: S,
    },
}
```

Generates
![image](https://user-images.githubusercontent.com/1502855/132259152-382f9517-c2a0-41d8-acd0-64e5993931fc.png)