build: extract LLVM distribution target handling This extracts the LLVM distribution target handling into a support module. Extraction will enable us to restructure the builds to support multiple distribution configurations (e.g. developer and user) to permit us to build the development package and the user package at once. llvm-svn: 363440 GitOrigin-RevId: 2b9f6caa71f732f2aebb9782916b859dd4ad3614