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)

tests/cpp17/test_cpp17.cpp

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2014 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// This is a sandbox for modeling C++17 code generator.
+// C++17 code generator: "flatc --cpp_std c++17".
+// Warning:
+// This is an experimental feature and could change at any time.
+
+#include "flatbuffers/flatbuffers.h"
+#include "flatbuffers/flexbuffers.h"
+#include "flatbuffers/idl.h"
+#include "flatbuffers/minireflect.h"
+#include "flatbuffers/registry.h"
+#include "flatbuffers/util.h"
+#include "test_assert.h"
+
+// Embed generated code into an isolated namespace.
+namespace cpp17 {
+#include "generated_cpp17/monster_test_generated.h"
+}  // namespace cpp17
+
+namespace cpp11 {
+#include "../monster_test_generated.h"
+}  // namespace cpp11
+
+int FlatBufferCpp17Tests() {
+  TEST_ASSERT(true);
+  return 0;
+}
+
+int main(int /*argc*/, const char * /*argv*/[]) {
+  InitTestEngine();
+
+  FlatBufferCpp17Tests();
+
+  if (!testing_fails) {
+    TEST_OUTPUT_LINE("C++17: ALL TESTS PASSED");
+  } else {
+    TEST_OUTPUT_LINE("C++17: %d FAILED TESTS", testing_fails);
+  }
+  return CloseTestEngine();
+}