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

[GHC]

#12599: Add Hadrian build artifacts to gitignore
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  bgamari
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       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:                    |
-------------------------------------+-------------------------------------

Comment (by snowleopard):

 Thanks Ben! I've moved generated includes outside of the tree. The
 remaining bits are:

 1. Files produced by `ghc-cabal` (e.g. `package-data.mk`)

 2. Package configuration folders such as `libraries/bootstrapping.conf`

 3. Program binaries

 If I understand correctly, we can close this ticket after fixing (1), as
 the Make build system also builds (2) and (3) in the tree.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12599#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler