MultiWayIf should start a new layout

Currently, MultiWayIf doesn't fully replace old case () of _ form. This outputs 3:

{-# LANGUAGE MultiWayIf #-}
x = case () of
    _ | False -> case () of
                _ | False -> 1
                  | False -> 2
      | True -> 3

main = print (x :: Int)

but this errors out:

{-# LANGUAGE MultiWayIf #-}
x = if | False -> if | False -> 1
                     | False -> 2
       | True -> 3

main = print (x :: Int)

because it parses like this:

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

Every other syntax introducing guards starts a layout; MultiWayIf's current behavior leads to subtle logic bugs for those unaware that MultiWayIf is special.