[GHC] #15018: Bump base's version number to 4.12.0.0 in HEAD

[GHC]

#15018: Bump base's version number to 4.12.0.0 in HEAD
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  libraries         |              Version:  8.5
  (other)                            |
      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 hvr):

 We still need the allow-newer functionality though, as we don't want to
 see packages w/ `base < 4.13` hitting Hackage before GHC 8.6.1 is
 released!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15018#comment:7>
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