[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
           Keywords:  newcomer       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 There are flags to control the typechecking and renaming trace (`-dtrace-
 tc`, `-dtrace-rn`) but the constraint solver trace can't be controlled in
 this way. It doesn't seem to always be emitted but it the behaviour should
 be unified with the other tracing functions.

 `traceTcS` should be implemented in terms of `traceOptTcRn` and a flag
 added to control the output.

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