This makes it possible to write

x = if | False -> if | False -> 1
                     | False -> 2
       | True -> 3

Layout normally inserts semicolons between declarations at the same
indentation level, so I added optional semicolons to the syntax for
guards in MultiWayIf syntax.  This is a bit of a hack, but the
alternative (a special kind of layout that doesn't insert semicolons)
seemed worse, or at least equally bad.