[GHC] #12687: Add a flag to control constraint solving trace

[GHC]

#12687: Add a flag to control constraint solving trace
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        Reporter:  mpickering        |                Owner:
            Type:  task              |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 It seems there is a flag `-ddump-cs-trace` but `csTraceTcM` implements
 it's own logic which should be unified with the logic in `traceOptTcRn`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12687#comment:1>
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