[GHC] #11026: Bump `base` version to 4.9.0.0

[GHC]

#11026: Bump `base` version to 4.9.0.0
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        Reporter:  hvr               |                Owner:  hvr
            Type:  task              |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  libraries/base    |              Version:
      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 Herbert Valerio Riedel <hvr@…>):

 In [changeset:"3238ef7f834ede18f076d4e8c72b9a119e4499b6/ghc"
 3238ef7f/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="3238ef7f834ede18f076d4e8c72b9a119e4499b6"
 Update unix submodule

 This is needed to prepare for #11026 as this update
 relaxes the upper bounds on `base` to allow for `base-4.9.0.0`
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11026#comment:9>
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