Issue Description:

Given this code:

let foo: String? = nil
if foo != nil {
    print(foo!)
}

Swift should warn and provide a fix-it to suggest that you write this instead:

let foo: String? = nil
if let foo = foo {
    print(foo)
}

(This would need to not provide a fixit if the variable is used as an l-value.)

I've seen a lot of code that does the former. It tends to make code more complicated, resulting in downstream nullability checks that aren't required. The compiler should help here.