Auto merge of #12539 - soruh:instanciate_empty_structs, r=Veykril

Automatically instaciate trivially instaciable structs in "Generate new" and "Fill struct fields"

As proposed in #12535 this PR changes the "Generate new" and "Fill struct fields" assist/diagnostic to instanciate structs with no fields and enums with a single empty variant.

For example:
```rust
pub enum Bar {
    Bar {},
}
struct Foo<T> {
    a: usize,
    bar: Bar,
    _phantom: std::marker::PhantomData<T>,
}
impl<T> Foo<T> {
    /* generate new */

    fn random() -> Self {
        Self { /* Fill struct fields */ }
    }
}
```

was previously:
```rust
impl<T> Foo<T> {
    fn new(a: usize, bar: Bar, _phantom: std::marker::PhantomData<T>) -> Self {
        Self { a, bar, _phantom }
    }

    fn random() -> Self {
        Self {
            a: todo!(),
            bar: todo!(),
            _phantom: todo!(),
        }
    }
}
```

and is now:
```rust
impl<T> Foo<T> {
  fn new(a: usize) -> Self {
      Self {
          a,
          bar: Bar::Bar {},
          _phantom: std::marker::PhantomData
      }
  }

  fn random() -> Self {
      Self {
          a: todo!(),
          bar: Bar::Bar {},
          _phantom: std::marker::PhantomData,
      }
  }
}
```

I'd be happy about any suggestions.

## TODO
   - [x]  deduplicate `use_trivial_constructor` (unclear how to do as it's used in two separate crates)
   - [x]  write tests

Closes #12535

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