[GHC] #12599: Add Hadrian build artifacts to gitignore

[GHC]

#12599: Add Hadrian build artifacts to gitignore
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8040             |  Differential Rev(s):  Phab:D2530
       Wiki Page:                    |
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Comment (by mgsloan):

 After running a hadrian build I see a clean working copy.  Perhaps this
 ticket can be closed?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12599#comment:12>
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