Auto merge of #56225 - alexreg:type_alias_enum_variants, r=petrochenkov

Auto merge of #56225 - alexreg:type_alias_enum_variants, r=petrochenkov

Implement RFC 2338, "Type alias enum variants"

This PR implements [RFC 2338](https://github.com/rust-lang/rfcs/pull/2338), allowing one to write code like the following.

```rust
#![feature(type_alias_enum_variants)]

enum Foo {
    Bar(i32),
    Baz { i: i32 },
}

type Alias = Foo;

fn main() {
    let t = Alias::Bar(0);
    let t = Alias::Baz { i: 0 };
    match t {
        Alias::Bar(_i) => {}
        Alias::Baz { i: _i } => {}
    }
}
```

Since `Self` can be considered a type alias in this context, it also enables using `Self::Variant` as both a constructor and pattern.

Fixes issues #56199 and #56611.

N.B., after discussing the syntax for type arguments on enum variants with @petrochenkov and @eddyb (there are also a few comments on the [tracking issue](https://github.com/rust-lang/rust/issues/49683)), the consensus seems to be treat the syntax as follows, which ought to be backwards-compatible.

```rust
Option::<u8>::None; // OK
Option::None::<u8>; // OK, but lint in near future (hard error next edition?)
Alias::<u8>::None; // OK
Alias::None::<u8>; // Error
```

I do not know if this will need an FCP, but let's start one if so.