[GHC] #11472: Remove CallStack CPP guards in GHC 8.2

[GHC]

#11472: Remove CallStack CPP guards in GHC 8.2
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           Reporter:  bgamari        |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  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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 GHC 7.10.1 didn't support `CallStack`, while its successors did. Yet in
 post-8.0 `master` (which needs to be buildable with the 7.10 series,
 including 7.10.1) references to CallStack have begun to sneak in to GHC.

 The first instance of this was 9d33adb6f352ad4e488067a8756928b3778920e0
 but there will likely be more. I've fixed in this instance in Phab:D1812
 by adding `#if` guards. These should be removed in GHC 8.2.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11472>
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