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

include/flatbuffers/base.h

@@ -70,6 +70,18 @@
 // Use the _MSC_VER and _MSVC_LANG definition instead of the __cplusplus  for compatibility.
 // The _MSVC_LANG macro reports the Standard version regardless of the '/Zc:__cplusplus' switch.
 
+#if defined(__GNUC__) && !defined(__clang__)
+  #define FLATBUFFERS_GCC (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
+#else
+  #define FLATBUFFERS_GCC 0
+#endif
+
+#if defined(__clang__)
+  #define FLATBUFFERS_CLANG (__clang_major__ * 10000 + __clang_minor__ * 100 + __clang_patchlevel__)
+#else
+  #define FLATBUFFERS_CLANG 0
+#endif
+
 /// @cond FLATBUFFERS_INTERNAL
 #if __cplusplus <= 199711L && \
     (!defined(_MSC_VER) || _MSC_VER < 1600) && \
@@ -236,12 +248,22 @@ template<typename T> FLATBUFFERS_CONSTEXPR inline bool IsConstTrue(T t) {
 }
 
 // Enable C++ attribute [[]] if std:c++17 or higher.
-#if (defined(__cplusplus) &&  (__cplusplus >= 201703L)) || \
-    (defined(_MSVC_LANG) &&  (_MSVC_LANG >= 201703L))
+#if ((__cplusplus >= 201703L) \
+    || (defined(_MSVC_LANG) &&  (_MSVC_LANG >= 201703L)))
   // All attributes unknown to an implementation are ignored without causing an error.
   #define FLATBUFFERS_ATTRIBUTE(attr) [[attr]]
+
+  #define FLATBUFFERS_FALLTHROUGH() [[fallthrough]]
 #else
   #define FLATBUFFERS_ATTRIBUTE(attr)
+
+  #if FLATBUFFERS_CLANG >= 30800
+    #define FLATBUFFERS_FALLTHROUGH() [[clang::fallthrough]]
+  #elif FLATBUFFERS_GCC >= 70300
+    #define FLATBUFFERS_FALLTHROUGH() [[gnu::fallthrough]]
+  #else
+    #define FLATBUFFERS_FALLTHROUGH()
+  #endif
 #endif
 
 /// @endcond