A bug in remapping code for network shares

Upstream issue: https://github.com/haskell/cabal/issues/6308#issuecomment-581895173

openFile goes through GHC and we seem to have a bug in the remapping code for network shares https://gitlab.haskell.org/ghc/ghc/blob/master/utils/fs/fs.c#L96 can you file an upstream ticket and assign it to me? I'll fix it tonight

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