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/namespace_test/NamespaceA/NamespaceB/StructInNestedNS.go

@@ -6,6 +6,27 @@ import (
 	flatbuffers "github.com/google/flatbuffers/go"
 )
 
+type StructInNestedNST struct {
+	A int32
+	B int32
+}
+
+func StructInNestedNSPack(builder *flatbuffers.Builder, t *StructInNestedNST) flatbuffers.UOffsetT {
+	if t == nil { return 0 }
+	return CreateStructInNestedNS(builder, t.A, t.B)
+}
+func (rcv *StructInNestedNS) UnPackTo(t *StructInNestedNST) {
+	t.A = rcv.A()
+	t.B = rcv.B()
+}
+
+func (rcv *StructInNestedNS) UnPack() *StructInNestedNST {
+	if rcv == nil { return nil }
+	t := &StructInNestedNST{}
+	rcv.UnPackTo(t)
+	return t
+}
+
 type StructInNestedNS struct {
 	_tab flatbuffers.Struct
 }