While LLVM's Clang has been supported on Windows for some time and there's been improvements made to the Clang Windows support over the years by various vendors, Microsoft is now working on Clang within Visual Studio.

New to Microsoft Visual Studio is support for building Android and iOS applications. When code for these alternative platforms is being used, Visual Studio is actually using Clang rather than Microsoft's Visual C++ compiler. It would appear this is likely how Microsoft is going to allow Visual Studio 2015 to target Linux. While Clang supports generating Windows binaries, Microsoft probably isn't yet ready to give up on their in-house compiler yet in favor of this open-source compiler backed by Apple, Google, and others.

Those interested in Microsoft's Clang usage for compiling iOS and Android apps, there's a new blog post over on the MSDN blog. Separately, Microsoft has been working on their own new .NET compiler based on LLVM.