The solution is to install a very specific build of MinGW. See http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-December/056..., where the recommended MinGW installation is "mingw-get-inst-20120426.exe". Then you must remove any other C compilers from your path and include this MinGW and llvm. On my Windows 7 x64 system I have three versions of Visual Studio and 2 other installations of MinGW, so I was careful to remove them all from my path. I created a pathclang.bat containing the following

    set PATH=C:\Windows\system32;c:\MinGW32\bin;c:\llvm\bin
    set CPATH=C:\MinGW32\include

and got clang 3.2 to work in 32-bit mode:

    pathclang.bat
    clang -m32 -o hello.exe hello.c
    hello.exe

Alas, it appears that 64-bit mode is currently unsupported on Windows.