This is similiar to the `-fwarn-noncanonical-monad-instances` warning
implemented via #11128, but applies to `Semigroup`/`Monoid` instead
and the `(<>)`/`mappend` methods (of which `mappend` is planned to move
out of `Monoid` at some point in the future being redundant and thus
error-prone).

This warning is contained in `-Wcompat` but not in `-Wall`.

This addresses #11150

Reviewed By: quchen

Differential Revision: https://phabricator.haskell.org/D1553