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

[GHC]

#12599: Add Hadrian build artifacts to gitignore
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 Andrey wrote requesting the following,

 > For top-level `.gitignore`:
 > {{{
 > #
 -------------------------------------------------------------------------------------
 > # Hadrian's stuff. Note, **/stageN/ folders will be relocated to _build/
 after
 > # resolving https://github.com/snowleopard/hadrian/issues/113
 > #
 -------------------------------------------------------------------------------------
 > _build/
 > hadrian/
 > **/stage?/
 > }}}
 >
 > For `.gitignore` in affected submodules (array, deepseq, directory,
 filepath,
 > hoopl, hpc, process, utils/hsc2hs):
 >
 > {{{
 > #
 -------------------------------------------------------------------------------------
 > # Hadrian's stuff. Note, stageN/ folders will be relocated to top-level
 _build/
 > # after resolving https://github.com/snowleopard/hadrian/issues/113
 > #
 -------------------------------------------------------------------------------------
 > stage?/
 > }}}

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