Rollup merge of #120822 - gurry:120756-terse-non-prim-cast-diag, r=petrochenkov

Emit more specific diagnostics when enums fail to cast with `as`

Fixes #120756

Changes this diagnostic reported in the issue:
```
error[E0605]: non-primitive cast: `Bad` as `u32`
  --> src/main.rs:18:10
   |
18 |     dbg!(bad as u32);
   |          ^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
```

to this:
```
error[E0605]: non-primitive cast: `Bad` as `u32`
  --> src/main.rs:18:10
   |
18 |     dbg!(bad as u32);
   |          ^^^^^^^^^^ an `as` expression can be used to convert enum types to numeric types only if the enum type is unit-only or field-less
   |
   = note: see https://doc.rust-lang.org/reference/items/enumerations.html#casting for more information
```

This change is only for enums. The diagnostic remains unchanged for all other cases.

Author: matthiaskrgr