As I understand it, it's not bringing Clang/LLVM's libc++ to Windows. The memory layout of, for example, std::string isn't decided by the compiler - it's decided by the library/header files. Both Clang and MSVC would be using the same MS C++ header files, so the only thing up to the compiler would anything that isn't specified in the header files, behind the scenes.
We're already using Clang with msbuild on Windows for native C11 support (come on, Microsoft!), courtesy of pre-built Clang for Windows binaries [1] - using the Microsoft C/C++ headers and linking against the Microsoft C/C++ runtimes.
There's already a plugin for MSVC that makes the Clang toolset available. VS 2013 (or maybe it was 2012?) decoupled the MSVC build chain/toolset from the IDE, making it possible to swap out which toolchain you're using from the project properties. Actually, it may have even been VS 2010, because I was using it to compile with ICC, but then again, I remember having a hell of a hard time so perhaps it was VS2012/2013.