If not overridden, AddClangSystemIncludeArgs's implementation is empty, so by default, no system include args are added to the clang driver. This means that invoking clang without the frontend must include a manual -I /usr/include flag, which is inconsistent behavior. Therefore, override and implement this method to match. Some boilerplate is also borrowed for handling of the other driver flags.
While we are here, also override and enable HasNativeLLVMSupport.