[GHC] #9623: Use Data.List.dropWhileEnd

[GHC]

#9623: Use Data.List.dropWhileEnd
-------------------------------------+-------------------------------------
              Reporter:  dfeuer      |            Owner:
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  libraries   |          Version:  7.8.3
  (other)                            |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by dfeuer):

 Replying to [comment:2 thomie]:
 > So `dropWhileEnd` was not added to base until version 4.5 (ghc 7.4), so
 to allow compilation with older versions of ghc, it would have to be
 conditionally imported and defined in the files you changed

 I'll get on that. Does GHC <7.4 compile GHC 7.9, or can I skip that for
 the stuff in the compiler?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9623#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler