Turn on -Werror=unused-private-field for Clang.

Change-Id: Ibabc1456baf190eba191ebe0b81e3d73a4acc98f

CMakeLists.txt

@@ -187,7 +187,7 @@ elseif(${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
       "${CMAKE_CXX_FLAGS} -std=c++0x -Wall -pedantic -Werror -Wextra -Wno-unused-parameter")
   set(FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wold-style-cast")
   if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.8)
-    list(APPEND FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wimplicit-fallthrough") # enable warning
+    list(APPEND FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wimplicit-fallthrough" "-Werror=unused-private-field") # enable warning
   endif()
   if(FLATBUFFERS_LIBCXX_WITH_CLANG)
     if(NOT "${CMAKE_SYSTEM_NAME}" MATCHES "Linux")

include/flatbuffers/flatbuffers.h

@@ -2180,6 +2180,7 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
       return (size > size_) ?  0 : size;
     #else
       // Must turn on FLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE for this to work.
+      (void)upper_bound_;
       FLATBUFFERS_ASSERT(false);
       return 0;
     #endif