Hello,
I built using https://llvm.org/docs/CMake.html in a separate directory as well as the build2 directory.
Sounds like you need to build the ubsan runtime. When you run cmake, make sure compiler-rt is part of LLVM_ENABLE_PROJECTS, as in: cmake -DLLVM_ENABLE_PROJECTS=llvm;clang;compiler-rt;lld;…
I believe it is not enabled by default.