Update comment search in ControlFlow::rewrite_pat_expr for for loops

[ytmimi]

Resolves #5009

For loops represented by a ControlFlow object use " in" as their connector.
In order to find the span where comments might be located, rustfmt
searches for the first uncommented occurrence of the word "in" within the
current span and adjusts it's starting point to look for comments right after that.
visually this looks like this:

    ruftfmt starts looking for comments here
            |
            V
    for x in /* ... */ 0..1 {}

This works well in most cases, however when the pattern also contains
the word "in", this leads to issues.

    ruftfmt starts looking for comments here
          |
          V
    for in_here in /* ... */ 0..1 {}
        -------
        pattern

In order to correctly identify the connector, the new approach first
updates the span to start after the pattern and then searches for the
first uncommented occurrence of "in".

[calebcartwright]

Thanks again! Another instance where you've got the right idea, though I think we can simplify the implementation a bit by taking advantage of existing spans directly from the AST.

[calebcartwright]

Thanks!