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/TableInNestedNS.go

@@ -6,6 +6,28 @@ import (
 	flatbuffers "github.com/google/flatbuffers/go"
 )
 
+type TableInNestedNST struct {
+	Foo int32
+}
+
+func TableInNestedNSPack(builder *flatbuffers.Builder, t *TableInNestedNST) flatbuffers.UOffsetT {
+	if t == nil { return 0 }
+	TableInNestedNSStart(builder)
+	TableInNestedNSAddFoo(builder, t.Foo)
+	return TableInNestedNSEnd(builder)
+}
+
+func (rcv *TableInNestedNS) UnPackTo(t *TableInNestedNST) {
+	t.Foo = rcv.Foo()
+}
+
+func (rcv *TableInNestedNS) UnPack() *TableInNestedNST {
+	if rcv == nil { return nil }
+	t := &TableInNestedNST{}
+	rcv.UnPackTo(t)
+	return t
+}
+
 type TableInNestedNS struct {
 	_tab flatbuffers.Table
 }