This requirement to check it out in a directory called 'extra' was never
documented for git users, so git users doing the obvious thing (just
clone, don't specify a name) will silently not get clang-tools-extra
Even reading the code is confusing because the
add_llvm_external_project macro appears to be used in place of the
add_subdirectory command, though the latter treats the second
placement argument as just a build directory name. So, anyone assuming
the same semantic with add_llvm_external_project would be confused.