Propagate restrictions against struct literals to the RHS of assignment

This prevents confusing errors when accidentally using an assignment in an `if` expression.  For example:

```rust
fn main() {
    let x = 1u;
    if x = x {
        println!("{}", x);
    }
}
```

Previously, this yielded:

```
test.rs:4:16: 4:17 error: expected `:`, found `!`
test.rs:4         println!("{}", x);
                         ^
```

With this change, it now yields:

```
test.rs:3:8: 3:13 error: mismatched types: expected `bool`, found `()` (expected bool, found ())
test.rs:3     if x = x {
                 ^~~~~
```

Closes issue #17283

r? @nick29581
It would probably be cleaner code to give the enum variants here int values and treat them like flags.