Flag warnings show intermediate hscpp filenames on SmartOS

[Alain O'Dea]

On SmartOS GHC flag warnings show up like this:
/tmp/ghc72341_0/ghc_1.hscpp:7:16: Warning:
    -fffi is deprecated: use -XForeignFunctionInterface or pragma {-#
LANGUAGE ForeignFunctionInterface #-} instead

It appears that this line in DriverPipeline leads to the correct filename
in warnings on Ubuntu, but not on SmartOS:
src_opts <- liftIO $ getOptionsFromFile dflags0 output_fn
https://github.com/ghc/ghc/blob/bb7f2e33197e667eb694bd1243f125c722a0a868/compiler/main/DriverPipeline.hs#L865

How does the intermediate hscpp filename get restored to a correct filename
if warnings are generated during a re-read of the pragmas after having
preprocessed the file?

Full investigation notes are here (rambling and unedited):
https://gist.github.com/AlainODea/98141991849093285c52
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