In a proper VS install, the path to cl.exe looks like:
...\VC\Tools\MSVC\14.11.25503\bin\HostX64\x64\cl.exe
In our automation, the path is just:
...\VC\bin\HostX64\x64\cl.exe
Clang tries to do some sanity-checking to make sure that the cl.exe it finds is the Microsoft compiler and not some other program. But the checks are a little too strict for us, so just look for "bin\Host*\*\cl.exe".
David Major