This seems preferable to printing two warnings per use of an unsupported option- one about not supporting it, and one about the compiler invocation not using it.
It also makes the '-Wno-' flag do what you mean with respect to those ignored options. Given that clang_ignored_f_Group are silently ignored, it seems obvious that the W flag is intended to suppress warnings about things that would otherwise print something, not arguments that are totally effectless.
Though why there are two groups at all is strange - the gcc group was added in r212805 when the clang_ignored_f_group already existed. I suppose it would have been incompatible to make the clang_ignored_f_group start to issue warnings.