Otherwise stand-alone build for tests fails due to missing include files.
A stand-alone build is a build performed outside LLVM source tree, e.g. out of split checkout, done against installed LLVM. In this case, with -DLLVM_MAIN_SRC_DIR= pointing explicitly to an external LLVM source tree needed for tests.
And how does it fail?
Actually, it seems that it doesn't fail anymore. I suppose it was fixed elsewhere. I'm sorry for the noise.