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)