[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)                           |
           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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 At the moment `placeHolderPunRhs` is defined as

 {{{#!hs
 placeHolderPunRhs :: LHsExpr RdrName
 -- The RHS of a punned record field will be filled in by the renamer
 -- It's better not to make it an error, in case we want to print it when
 debugging
 placeHolderPunRhs = noLoc (HsVar pun_RDR)

 pun_RDR  = mkUnqual varName (fsLit "pun-right-hand-side")
 }}}

 Rather use a type of `PostRn id (LHsExpr id)` where this is used.

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