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)

src/idl_parser.cpp

@@ -627,12 +627,6 @@ CheckedError Parser::ParseType(Type &type) {
             "length of fixed-length array must be positive and fit to "
             "uint16_t type");
       }
-      // Check if enum arrays are used in C++ without specifying --scoped-enums
-      if ((opts.lang_to_generate & IDLOptions::kCpp) && !opts.scoped_enums &&
-          IsEnum(subtype)) {
-        return Error(
-            "--scoped-enums must be enabled to use enum arrays in C++\n");
-      }
       type = Type(BASE_TYPE_ARRAY, subtype.struct_def, subtype.enum_def,
                   fixed_length);
       NEXT();