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.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/test/ui/qualified/qualified-path-params-2.stderr b/src/test/ui/qualified/qualified-path-params-2.stderr
index 91c3d704cb9..4e073841b97 100644
--- a/src/test/ui/qualified/qualified-path-params-2.stderr
+++ b/src/test/ui/qualified/qualified-path-params-2.stderr
@@ -1,8 +1,8 @@
-error[E0109]: type parameters are not allowed on this type
+error[E0109]: type arguments are not allowed on this entity
   --> $DIR/qualified-path-params-2.rs:18:26
    |
 LL | type A = <S as Tr>::A::f<u8>;
-   |                          ^^ type parameter not allowed
+   |                          ^^ type argument not allowed
 
 error[E0223]: ambiguous associated type
   --> $DIR/qualified-path-params-2.rs:18:10