Introduce TcRnWithHsDocContext

Currently GHC uses withHsDocContext to attach a HsDocContext to an error message. Here's an example:

addErr $ mkTcRnUnknownMessage $ mkPlainError noHints (withHsDocContext ctxt msg)

The problem with this is that this approach only works with TcRnUnknownMessage. But could we attach an HsDocContext to a structured error message? In !9227 (comment 459522), I proposed that we add the following constructor:

data TcRnMessage where
  ...
  TcRnWithHsDocContext :: !HsDocContext -> !TcRnMessage -> TcRnMessage
  ...

Perhaps this is not the best design possible, it is better than what we have now, so I'm going to follow through with this.