message: build: allow the user to specify the llvm tools
  This is needed for cross-compiling the toolchain to Windows ARM64 on
  Windows x64 using Visual Studio which does not permit the NATIVE
  sub-build to work properly.
diff: @@ -32,6 +32,9 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
  endif()
  set(LLVM_DEFAULT_EXTERNAL_LIT "${LLVM_TOOLS_BINARY_DIR}/${lit_file_name}" CACHE PATH "Path to llvm-lit")
  + if(LLVM_TABLEGEN)
  + set(LLVM_TABLEGEN_EXE ${LLVM_TABLEGEN})
  + else()
  if(CMAKE_CROSSCOMPILING)
  set(LLVM_NATIVE_BUILD "${LLDB_PATH_TO_LLVM_BUILD}/NATIVE")
  if (NOT EXISTS "${LLVM_NATIVE_BUILD}")
  @@ -57,6 +60,7 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
  find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
  NO_DEFAULT_PATH)
  endif()
  + endif()
  # They are used as destination of target generators.
  set(LLVM_RUNTIME_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin)
  