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)

docs/source/Compiler.md

@@ -132,6 +132,11 @@ Additional options:
     std::string from Flatbuffers, but (char* + length). This allows efficient
 	construction of custom string types, including zero-copy construction.
 
+-   `--cpp-std CPP_STD` : Generate a C++ code using features of selected C++ standard.
+     Supported `CPP_STD` values:
+    * `c++0x` - generate code compatible with old compilers (VS2010).
+    * `c++11` - use C++11 code generator (default);
+
 -   `--object-prefix` : Customise class prefix for C++ object-based API.
 
 -   `--object-suffix` : Customise class suffix for C++ object-based API.