Closures in enum discriminants error when they shouldn't

This [used to work](https://godbolt.org/g/YUf5HC) (regression from 1.14 stable) but now it doesn't:
```rust
enum Functions {
    Square = (|x:i32| { }, 42).1,
}
```
In #50623 the error less bad and some `FIXME(#50689)` are left in the code with hints, but still doesn't work. Issue #50688 seems related.