Unable to format ":" chains containing do blocks

Ormolu is currently unable to format this (`Parsing of formatted code failed`):

```haskell
module A (a) where

b :: [Int]
b =
  1 :
    do
      2
     : do
        3
       : do
          [4]
```

With `--unsafe`, we get the following formatted output:

```haskell
module A (a) where

b :: [Int]
b =
  1 :
  do
    2 :
  do
    3 :
    do
      [4]
```