Flag consecutive `let/var` usages in optional binding

**Preferred**

``` swift
if var a = a, b = b, c = c where c != 0 {
    print("(a + b) / c = \((a + b) / c)")     // (a + b) / c = 5
}
```

**Not Preferred**

``` swift
if var a = a, var b = b, c = c where c != 0 {
    print("(a + b) / c = \((a + b) / c)")     // (a + b) / c = 5
}

if let a = a, b = b, let c = c where c != 0 {
    print("(a + b) / c = \((a + b) / c)")     // (a + b) / c = 5
}
```
