Preprocess code before checking safety

[favonia]

Apparently GHC checks safety (as safe Haskell) before preprocessing. This is sometimes undesirable; for example:

#if __GLASGOW_HASKELL__ >= 702
import Foreign.Safe
#else
import Foreign
#endif

Not sure if preprocessing will break the design of safe Haskell, but if it won't I hope GHC can accept this code as safe.

[favonia]

I am sorry but I cannot reproduce this error anymore. Perhaps I messed up something before.