[rustdoc] Show enum discrimant if it is a C-like variant
Fixes https://github.com/rust-lang/rust/issues/101337.
We currently display values for associated constant items in traits:
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:
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`
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