Ideally, we'd have the more general

    instance (MonadTrans t, Monad m, HasDynFlags m) => HasDynFlags (t m) where
        getDynFlags = lift getDynFlags

definition. However, that one would overlap with the `HasDynFlags (GhcT m)`
instance. Instead we define instances for a couple of common Monad
transformers explicitly in order to avoid nasty overlapping instances.

This is a preparatory refactoring for #10874

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D1581