This builds on top of D77184. With this, I can rename my build directory
to a different name and bin/llvm-lit ../llvm-project/clang/test ../llvm-project/llvm/test
I haven't tried copying the build dir to a different machine to run tests there yet,
but I tried something like it a few months ago and it worked back then.
- Make configure_lit_site_cfg() store the main / generated config pair interleaved in the LLVM_LIT_CONFIG_FILES list and postpone converting it to python code to llvm-lit's CMakeList.
- Pull the relpath code into a new function make_paths_relative() and call that in llvm-lit's CMakeList, prior to converting the list to python code.
- Pull the path() function into a variable and use that in llvm-lit's CMakeList too.