compnerd (CC’d) did the initial work to port libc++ to Windows (including adding support for native Windows threading). EricWF (also CC’d) continued that work, particularly around tests and vcruntime support. I’ve done some fix-ups here and there.
Last I checked, libc++ on Windows x86 compiled successfully with clang-cl, and only failed around 200 tests (out of over 5000). There’s documentation for building and running tests on Windows at http://libcxx.llvm.org/docs/BuildingLibcxx.html#experimental-support-for-windows. I was maintaining a Google Doc tracking the test failures at https://docs.google.com/spreadsheets/d/1Yonp8WidaIBb9mmzy6aV92wbjBYEargyrlIYc75r3Gk/edit#gid=0, though it’s probably fairly out of date now. EricWF also set up an appveyor bot at https://ci.appveyor.com/project/EricWF/libcxx.
One known issue is a lack of locale support (https://bugs.llvm.org/show_bug.cgi?id=31516). I’ll be working more on addressing that (and the remaining test failures) over the next couple of months.
cl support hasn’t been an explicit goal for any of us yet. It would definitely be nice to have though, and I’m aware of the work you’ve been doing on that front.