[GHC] #12224: Replace placeHolderPunRhs with a PlaceHolder value

[GHC]

#12224: Replace placeHolderPunRhs with a PlaceHolder value
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        Reporter:  alanz             |                Owner:  alanz
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
  (Parser)                           |
      Resolution:                    |             Keywords:
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 alanz):

 It seems that to do this I need to have a `Functor` instance for `PostRn
 id`, which does not seem possible.

 Any pointers?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12224#comment:1>
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