Fix high certainty warnings from PVS-studio (#5115)

* Fix high certainty warnings from PVS-studio

- Introduced FLATBUFFERS_ATTRIBUTE macro to use [[attribute]] if modern C++ compiler used

* Update the note about __cplusplus usage in the MSVC

include/flatbuffers/base.h

@@ -59,6 +59,15 @@
 // Clang 3.4 and later implement all of the ISO C++ 2014 standard.
 // http://clang.llvm.org/cxx_status.html
 
+// Note the MSVC value '__cplusplus' may be incorrect:
+// The '__cplusplus' predefined macro in the MSVC stuck at the value 199711L,
+// indicating (erroneously!) that the compiler conformed to the C++98 Standard.
+// This value should be correct starting from MSVC2017-15.7-Preview-3.
+// The '__cplusplus' will be valid only if MSVC2017-15.7-P3 and the `/Zc:__cplusplus` switch is set.
+// Workaround (for details see MSDN):
+// 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.
+
 /// @cond FLATBUFFERS_INTERNAL
 #if __cplusplus <= 199711L && \
     (!defined(_MSC_VER) || _MSC_VER < 1600) && \
@@ -224,6 +233,15 @@ template<typename T> FLATBUFFERS_CONSTEXPR inline bool IsConstTrue(T t) {
   return !!t;
 }
 
+// Enable of std:c++17 or higher.
+#if (defined(__cplusplus) &&  (__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]] - will be enabled in a future release
+#else
+  #define FLATBUFFERS_ATTRIBUTE(attr)
+#endif
+
 /// @endcond
 
 /// @file