Suggest potentially missing value in empty conditional expression arms with inferred type

[estebank]

When given

fn d(opt_str: Option<String>) {
    let s = if let Some(s) = opt_str {
        //~^ ERROR mismatched types
    } else {
        String::new()
    };
}

we should mention that s might be missing in the if branch of the expression, like we do for

fn d(opt_str: Option<String>) {
    let s: String = if let Some(s) = opt_str {
        //~^ ERROR mismatched types
    } else {
        String::new()
    };
}

#98784 (comment)

[compiler-errors]

Actually this aligns perfectly with a refactoring of IfExpressionCause I was looking at last night...

[compiler-errors]

Since we already suggest removing semicolon bidirectionally i.e. if a { b; } else { c } and if a { b } else { c; }, we should be able to do the same with the return binding suggestion too.

@rustbot claim