Maybe I'm misinterpreting your language but I don't see how pitchforks will help. There are simply not enough Windows users among clang/llvm developers and none of the companies that are invested in clang/llvm use it for Windows technology. If Windows users want a working C/C++ compiler they will have to stand up and actually contribute, but that seems to be a foreign concept to many of the developers coming from that platform (purely anecdotal and I have no real evidence). It is not like anybody is actively trying to make clang/llvm not work under Windows.
On the other side of the fence Microsoft has repeatedly stated that they are not interested in supporting more of ISO C than they already do and want to focus on C++ (even going so far as to recommend the C++ compiler for developing C code). And apparently that "focus" on C++ hasn't even helped them overtake or even get even with their free (beer and freedom both) alternatives. That goes a long way to show in what state their compiler division is and so I wouldn't even bet on getting anything halfway decent should they commit to C11. That is not to say that there are highly competent people on that team, but there is simply to much to do and too many old mistakes to rectify first.