SysTools.removeTmpFiles:
  - never delete source files with a DriverPhases.haskellish_user_src_file
    file extension & loudly complain should the compiler attempt to do so.

    This is a protective measure against bugs elsewhere in the driver pipeline
    (cf., 'ghc-6.0 --make' deleting input files if specified using backward
     instead of forward slashes under win32.)