Visual Studio 2015 Update 3 + v140_clang_3_7 does not succeed
It seems since VS2015U3 this toolchain is named now 'v140_clang_c2'. Probably it will stay as 'vXXX_clang_c2'.
And clang compiler (3.8.0 at the moment) is now under:
c:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\ClangC2\
c:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\ClangC2\bin\amd64\clang.exe
c:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\ClangC2\bin\x86\clang.exe