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

[GHC]

#12599: Add Hadrian build artifacts to gitignore
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        Reporter:  bgamari           |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       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:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 As indicated in the comments, these entries would disappear when
 https://github.com/snowleopard/hadrian/issues/113 is resolved.
 Consequently, I'm not entirely sure this is something we really want to do
 since it will pollute a large number of repositories outside of GHC. I'm
 going to see what it would take to resolve the issue in hopes that we can
 avoid adding the gitignores.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12599#comment:1>
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