Swift/Ninja: Relax split build requirements

If the Swift compiler is building with more than one thread, it will always emit multiple objects, regardless of the compilation mode. To that end, we only need to actually track the compilation mode if the number of threads is set to zero in order to accurately model the split build model. Given that we have tied -j and -num-threads to the same variable, and that -j cannot be zero, it would be quite challenging for someone to override this and break things at the moment.