Nearly all C++ features with Clang should now work well under Windows, LLVM/Clang can self-host using the Clang-cl component, and the Chrome/Firefox browsers can now successfully compile in the Clang Windows mode. There's also compatibility with the Microsoft record layout and as of this week clang-cl is compatible with Visual Studio 2013 and Clang supports all calling conventions up through Visual Studio 2012. Run-Time Type Information (RTTI) support was also completed.
Those interested in more information on the native Windows build support for LLVM's Clang C/C++ compiler can find out more via the LLVM blog. These latest improvements can be found in LLVM 3.5 that should be released in August.