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/flatc_main.cpp

@@ -30,10 +30,20 @@ static void Error(const flatbuffers::FlatCompiler *flatc,
                   const std::string &err, bool usage, bool show_exe_name) {
   if (show_exe_name) { printf("%s: ", g_program_name); }
   printf("error: %s\n", err.c_str());
-  if (usage) { printf("%s", flatc->GetUsageString(g_program_name).c_str()); }
+  if (usage && flatc) { printf("%s", flatc->GetUsageString(g_program_name).c_str()); }
   exit(1);
 }
 
+namespace flatbuffers {
+void LogCompilerWarn(const std::string &warn) {
+  Warn(static_cast<const flatbuffers::FlatCompiler *>(nullptr), warn, true);
+}
+void LogCompilerError(const std::string &err) {
+  Error(static_cast<const flatbuffers::FlatCompiler *>(nullptr), err, false,
+        true);
+}
+}  // namespace flatbuffers
+
 int main(int argc, const char *argv[]) {
   // Prevent Appveyor-CI hangs.
   flatbuffers::SetupDefaultCRTReportMode();