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)

grpc/build_grpc.sh

@@ -5,7 +5,7 @@ grpc_1_15_1_githash=1a60e6971f428323245a930031ad267bb3142ba4
 function build_grpc () {
   git clone https://github.com/grpc/grpc.git google/grpc
   cd google/grpc
-  git checkout ${grpc_1_15_1_githash} 
+  git checkout ${grpc_1_15_1_githash}
   git submodule update --init
   make
   make install prefix=`pwd`/install