Add flatc '--cpp_std' switch (#5656)

* Add flatc '--cpp_std' switch and sandbox for C++17 code generator

- Added 'flac --cpp_std legacy' for compatibility with old compilers (VS2010);
- Added experimental switch 'flac --cpp_std c++17' for future development;
- Added C++17 sandbox test_cpp17.cpp;
- C++ code generator generates enums with explicit underlying type to avoid problems with the forward and backward schema compatibility;
- Adjusted CMakeLists.txt, CI and generate code scripts to support of introduced '--cpp_std';

* Fix --cpp_std values: c++0x, c++11, c++17

* Add 'cpp::CppStandard' enum

* Add testing engine into test_cpp17

* Rebase to upstream/master

* Set default '--cpp-std C++0x'

* Fix code generation (--cpp_std C++11) in CMakeLists.txt

- Fix dependency declaration of grpctest target

* Revert --cpp-std for the tests from explicit C++11 to flatc default value (C++0x)

include/flatbuffers/reflection_generated.h

@@ -97,7 +97,7 @@ inline const char * const *EnumNamesBaseType() {
 }
 
 inline const char *EnumNameBaseType(BaseType e) {
-  if (e < None || e > Array) return "";
+  if (flatbuffers::IsOutRange(e, None, Array)) return "";
   const size_t index = static_cast<size_t>(e);
   return EnumNamesBaseType()[index];
 }