[GHC] #11139: Add Semigroup warnings

[GHC]

#11139: Add Semigroup warnings
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        Reporter:  quchen            |                Owner:
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1539
       Wiki Page:                    |
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Changes (by quchen):

 * status:  new => patch
 * differential:   => Phab:D1539


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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11139#comment:1>
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