Report error for C++ style comment ending with a backslash like (#2775)

// ... some content ... \
On Clang or AppleClang
PaddlePaddle sets -Werror=comment

cmake/clang_4_0_flags.cmake

@@ -16,6 +16,7 @@
 
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=return-type")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=inconsistent-missing-override")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=comment")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic-errors")
 
  # whitelist errors here