By default xcodebuld pulls the tip HEAD of LLVM and Clang inside LLDB for building, unlike cmake/ninja. What I am looking for is a xcodebuild command-line parameter that can point to the custom llvm/clang source path.
From build-llvm.pl script, it seems like this facility already exists, and it is possible to point to custom location using SCRIPT_INPUT_FILE_0 environment variable.
Eg. Directory structure: