Rustfmt incorrectly strips comments when removing empty where clause

Empty where clauses are semantically valid Rust. However rustfmt incorrectly removes them and comments.

Before:

trait Foo {
    fn bar(&self)
    where
    //     Self: Bar
    // Some comment
    ;
}

fn foo<T>()
where
//     T: Bar,
// Some comment
{
    println!("foo");
}

After:

trait Foo {
    fn bar(&self);
}

fn foo<T>() {
    println!("foo");
}

rustfmt 1.4.25-stable (0f29ff6d 2020-11-11)
I also tested it on master a97fd770e322714933f6f05b6244303c50199dd3

Start by reproducing the issue with the two Rust snippets in the report and trace rustfmt's handling of empty where clauses. The fix is complete when formatting preserves the comments in both examples while still handling the empty clauses correctly; add regression coverage where the existing formatter tests for this behavior are found.