Convert restriction enum into bitflags

This makes having multiple restrictions at once cleaner
Propagate restrictions against struct literals to the RHS of assignments

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
Add regression test for issue #17283