[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:  closed
        Priority:  highest           |            Milestone:  8.0.1
       Component:  libraries/base    |              Version:
      Resolution:  fixed             |             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:"5065cf490ccdf03dda1e148afb979b8838e6c32e/ghc"
 5065cf49/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="5065cf490ccdf03dda1e148afb979b8838e6c32e"
 base: Update `@since 4.8.2` annotations (re #11026)

 This missed to perform in f8ba4b55cc3a061458f5cfabf17de96128defbbb
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11026#comment:13>
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