Gracefully handle missing comma between struct style enum variants

[estebank]

The following code should gracefully recover from parse error, provide an appropriate suggestion and continue to typechk:

enum Foo {
    Bar {
    }
    /// doc
    Qux {
    }
    Quz,
}
fn main() {
    let x: usize = Foo::Quz;
}

error: expected one of `,` or `}`, found `/// doc`
 --> src/lib.rs:4:5
  |
3 |     }
  |      - help: missing comma here
4 |     /// doc
  |     ^^^^^^^ unexpected token

error: expected one of `,` or `}`, found `Quz`
 --> src/lib.rs:7:5
  |
6 |     }
  |      - help: missing comma here
7 |     Quz,
  |     ^^^ unexpected token

error[E0308]: mismatched types
  --> src/main.rs:10:20
   |
10 |     let x: usize = Foo::Quz;
   |                    ^^^^^^^^ expected usize, found enum `Foo`
   |
   = note: expected type `usize`
              found type `Foo`

---

If this could be generalized to all "unexpected token" cases, even better, but that might get tricky.

CC #48724.

[estebank]

This should also handle the tuple variants case (enum E { V(usize) Foo }).