[GHC] #14601: Remove uses of unsafeGlobalDynFlags for -fopt-coercions

[GHC]

#14601: Remove uses of unsafeGlobalDynFlags for -fopt-coercions
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           Reporter:  bgamari        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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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 Currently the `-fopt-coercions` flag finds its way to
 `OptCoercion.optCoercion` by way of the terrible `unsafeGlobalDynFlags`.
 In general it's nice to minimize the uses of `unsafeGlobalDynFlags`.
 However, it's not entirely clear that passing a `DynFlags` to
 `optCoercion` is worthwhile in this case as `-fopt-coercion` is
 essentially a debug flag. Leaving this here for later consideration.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14601>
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