Implement warnings for Semigroups as parent of Monoid

This patch is similar to the AMP patch (#8004), which offered two
functions:

  1. Warn when an instance of a class has been given, but the type does
     not have a certain superclass instance
  2. Warn when top-level definitions conflict with future Prelude names

These warnings are issued as part of the new `-Wcompat` warning group.

Reviewers: hvr, ekmett, austin, bgamari

Reviewed By: hvr, ekmett, bgamari

Subscribers: ekmett, thomie

Differential Revision: https://phabricator.haskell.org/D1539

GHC Trac Issues: #11139