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".

diff --git a/clang/lib/Driver/ToolChains/MSVC.cpp b/clang/lib/Driver/ToolChains/MSVC.cpp
index 7978a6941cb..0159e89fa27 100644
--- a/clang/lib/Driver/ToolChains/MSVC.cpp
+++ b/clang/lib/Driver/ToolChains/MSVC.cpp
@@ -152,8 +152,7 @@ static bool findVCToolChainViaEnvironment(std::string &Path,
         // path components with these prefixes when walking backwards through
         // the path.
         // Note: empty strings match anything.
-        llvm::StringRef ExpectedPrefixes[] = {"",     "Host",  "bin", "",
-                                              "MSVC", "Tools", "VC"};
+        llvm::StringRef ExpectedPrefixes[] = {"", "Host",  "bin"};
 
         auto It = llvm::sys::path::rbegin(PathEntry);
         auto End = llvm::sys::path::rend(PathEntry);