clang --help tells about -cxx-abi parameter, but it not works:
$ clang -cxx-abi microsoft foo.cpp
clang: warning: argument unused during compilation: '-cxx-abi microsoft'
Instead of it, you need to pass this option to CC1 directly:
$ clang -Xclang -cxx-abi -Xclang microsoft
But this is not equal with help. So how to fix?
As I recall, this happened when the driver Option tables and the -cc1 Option tables were merged; the right fix is probably not to list -cc1 options with no driver support in the driver's help :).
As for making -cxx-abi a driver option: the alternate C++ ABI code isn't production-ready yet, and it probably won't be for a while now.