message: build: normalise the path before use (NFC)
  Normalise the path before using it for the inclusion, otherwise Windows
  paths can break.
diff: @@ -72,6 +72,7 @@ endif()
  # We append the directory in which LLVMConfig.cmake lives. We expect LLVM's
  # CMake modules to be in that directory as well.
  +file(TO_CMAKE_PATH ${LLVM_DIR} LLVM_DIR)
  list(APPEND CMAKE_MODULE_PATH "${LLVM_DIR}")
  list(APPEND CMAKE_MODULE_PATH "${SWIFT_CMAKE_DIR}")
  include(AddLLVM)
  