msan fails to get configured

In the past, when I’ve seen this error, it’s because I didn’t have g++ or clang++ installed

Try “which c++” or “which g++”


How would "which g++" deal with


  is not a full path to an existing compiler tool.


Note that I did not run CMake on msan. I ran CMake on the toplevel
"llvm" directory. Msan's CMake was invoked *during the build*.