Repair fallthrough was broken by d4493165 (#5115) (#5139)

- GCC: fixed broken `fallthrough` (checked with 7.3 and 8.2)
- Clang: added `fallthrough` support
- Clang: added `-Wimplicit-fallthrough` checking

CMakeLists.txt

@@ -152,6 +152,7 @@ set(FlatBuffers_GRPCTest_SRCS
 if(EXISTS "${CMAKE_TOOLCHAIN_FILE}")
   # do not apply any global settings if the toolchain
   # is being configured externally
+  message(STATUS "Using toolchain file: ${CMAKE_TOOLCHAIN_FILE}.")
 elseif(APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -stdlib=libc++")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -Werror -Wextra -Wno-unused-parameter")
@@ -185,6 +186,9 @@ elseif(${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
   set(CMAKE_CXX_FLAGS
       "${CMAKE_CXX_FLAGS} -std=c++0x -Wall -pedantic -Werror -Wextra -Wno-unused-parameter")
   set(FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wold-style-cast")
+  if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.8)
+    list(APPEND FLATBUFFERS_PRIVATE_CXX_FLAGS "-Wimplicit-fallthrough") # enable warning
+  endif()
   if(FLATBUFFERS_LIBCXX_WITH_CLANG)
     if(NOT "${CMAKE_SYSTEM_NAME}" MATCHES "Linux")
       set(CMAKE_CXX_FLAGS