[LLVM 4.0] Fix CreateCompileUnit
This is largely identical to @dylanmckay's [patch](https://github.com/dylanmckay), except that it doesn't try to use `file_metadata()`. I don't think that is necessary because we don't want the compile unit to be added to `debug_context.created_files`, though I'd like confirmation from someone who knows for sure. If that is needed, I can modify `file_metadata_()` so that it can be used from `compile_unit_metadata()`.
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