CMake build: CommandLine Error: Option 'help-list' registered more than once!

With the CMake build, both with clang-8/llvm-8 and clang-9/llvm-9, I get the following message, and the code model doesn't work (F2 works, but highlighting doesn't work, and refactoring with Alt-Enter is broken):

: CommandLine Error: Option 'help-list' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options
Empty filename passed to function
Empty filename passed to function
Empty filename passed to function
Empty filename passed to function

The system is debian sid, cmake version is 9c31d83aa2a3d3f5921f4a5a559e126e285b96c5, LLVM/Clang are 9.0.0 from the system.