I was not aware of intentional instability of llvm and clang APIs. That certainly causes havoc.
Thanks for the suggestion about building separately ROOT’s llvm and clang. I suppose I can count on ROOT build system checking compatibility between given build of llvm and clang and the current version of ROOT, right? So that I will hit cmake error if there is some development in llvm or clang that new ROOT version needs, as opposite to hitting a compilation error few minutes in the build process.