having mutable pointer to comptime-only function type does not compile error

### Zig Version

0.14.0-dev.2987+183bb8b08

### Steps to Reproduce and Observed Behavior

```zig
test {
    var x: u32 = 4;
    const y: *fn () void = @ptrCast(&x);
    _ = y;
}
```

```
❯ ./zig test ./test.zig
All 1 tests passed.
```


### Expected Behavior

compile error