Auto merge of #116142 - GuillaumeGomez:enum-variant-display, r=fmease

[rustdoc] Show enum discrimant if it is a C-like variant

Fixes rust-lang/rust#101337.

We currently display values for associated constant items in traits:

![image](https://github.com/rust-lang/rust/assets/3050060/03e566ec-c670-47b4-8ca2-b982baa7a0f4)

And we also display constant values like [here](file:///home/imperio/rust/rust/build/x86_64-unknown-linux-gnu/doc/std/f32/consts/constant.E.html).

I think that for coherency, we should display values of C-like enum variants.

With this change, it looks like this:

![image](https://github.com/rust-lang/rust/assets/3050060/b53fbbe0-bdb1-4289-8537-f2dd4988e9ac)

As for the display of the constant value itself, I used what we already have to keep coherency.

We display the C-like variants value in the following scenario:
 1. It is a C-like variant with a value set => all the time
 2. It is a C-like variant without a value set: All other variants are C-like variants and at least one them has its value set.

Here is the result in code:

```rust
// Ax and Bx value will be displayed.
enum A {
    Ax = 12,
    Bx,
}

// Ax and Bx value will not be displayed
enum B {
    Ax,
    Bx,
}

// Bx value will not be displayed
enum C {
    Ax(u32),
    Bx,
}

// Bx value will not be displayed, Cx value will be displayed.
#[repr(u32)]
enum D {
    Ax(u32),
    Bx,
    Cx = 12,
}
```

r? `@notriddle`

Author: bors