The MonadFail proposal implemented so far via #10751 only warns about
missing `MonadFail` instances based on existence of failible pattern
matches in `do`-blocks.

However, based on the noncanonical Monad warnings implemented via #11150
we can provide a different mechanism for detecting missing `MonadFail`
instances quite cheaply. That is, by checking for canonical `fail` definitions.

In the case of `Monad`/`MonadFail`, we define the canonical implementation of
`fail` to be such that the soft-deprecated method shall (iff overridden) be
defined in terms of the non-deprecated method. Consequently, in case of
`MonadFail`, the `Monad(fail)` method shall be defined as alias of
the `MonadFail(fail)` method.

This allows us at some distant point in the future to remove `fail` from
the `Monad` class, while having GHC ignore/tolerate such literal canonical
method definitions.

Reviewed By: bgamari, RyanGlScott

Differential Revision: https://phabricator.haskell.org/D1838