[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:                 |           Version:
  libraries/base                     |
           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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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11026>
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