[GHC] #12398: Expose 'withCleanupSession' as a replacement for 'defaultCleanupHandler'

[GHC]

#12398: Expose 'withCleanupSession' as a replacement for 'defaultCleanupHandler'
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        Reporter:  DanielG           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       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 DanielG):

 I have my own `GhcT`/`runGhcT` variant that does all the initialization.
 Ideally a function doing just the important initialization bits would be
 exported too but it's simple enough that it doesn't bother me either way.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12398#comment:4>
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