EPA : reconsider the DeltaPos encoding

This is a follow up from !2418 (comment 338141), and #19579

At the moment the exact printing types include a DeltaPos defined as

data DeltaPos =
  DP
    { deltaLine   :: !Int,
      deltaColumn :: !Int
    } deriving (Show,Eq,Ord,Data)

But it actually has a different meaning depending on whether the delta is on the same line or a different one.

Replace this with

data DeltaPos
  = SameLine { deltaColumn :: !Int }
  | DifferentLine { deltaLine :: !Int, startColumn :: !Int }