See RFC to eliminate "REQUIRES: no*"

I've posted an RFC on llvm-dev to eliminate Lit feature keywords
that start with "no" "not_" or "non-" (all the negative ones I
can find), because this can mislead people into thinking other
feature keywords can have "no" etc. in front automatically.

Please reply to the llvm-dev thread, to avoid cross-posting
hassles.
Thanks,
--paulr