Many LLVM-based tools already support response files (i.e. files containing a list of options, specified with '@'). This change simply updates the documentation and help text for some of these tools to include it. I haven't attempted to fix all tools, just a selection that I am interested in.
I've taken the opportunity to add some tests for --help behaviour, where they were missing. We could expand these tests, but I don't think that's within scope of this patch.