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
 }

tests/namespace_test/NamespaceA/NamespaceB/StructInNestedNS.py

@@ -23,3 +23,34 @@ def CreateStructInNestedNS(builder, a, b):
     builder.PrependInt32(b)
     builder.PrependInt32(a)
     return builder.Offset()
+
+
+class StructInNestedNST(object):
+
+    # StructInNestedNST
+    def __init__(self):
+        self.a = 0  # type: int
+        self.b = 0  # type: int
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        structInNestedNS = StructInNestedNS()
+        structInNestedNS.Init(buf, pos)
+        return cls.InitFromObj(structInNestedNS)
+
+    @classmethod
+    def InitFromObj(cls, structInNestedNS):
+        x = StructInNestedNST()
+        x._UnPack(structInNestedNS)
+        return x
+
+    # StructInNestedNST
+    def _UnPack(self, structInNestedNS):
+        if structInNestedNS is None:
+            return
+        self.a = structInNestedNS.A()
+        self.b = structInNestedNS.B()
+
+    # StructInNestedNST
+    def Pack(self, builder):
+        return CreateStructInNestedNS(builder, self.a, self.b)

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
 }

tests/namespace_test/NamespaceA/NamespaceB/TableInNestedNS.py

@@ -30,3 +30,35 @@ class TableInNestedNS(object):
 def TableInNestedNSStart(builder): builder.StartObject(1)
 def TableInNestedNSAddFoo(builder, foo): builder.PrependInt32Slot(0, foo, 0)
 def TableInNestedNSEnd(builder): return builder.EndObject()
+
+
+class TableInNestedNST(object):
+
+    # TableInNestedNST
+    def __init__(self):
+        self.foo = 0  # type: int
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        tableInNestedNS = TableInNestedNS()
+        tableInNestedNS.Init(buf, pos)
+        return cls.InitFromObj(tableInNestedNS)
+
+    @classmethod
+    def InitFromObj(cls, tableInNestedNS):
+        x = TableInNestedNST()
+        x._UnPack(tableInNestedNS)
+        return x
+
+    # TableInNestedNST
+    def _UnPack(self, tableInNestedNS):
+        if tableInNestedNS is None:
+            return
+        self.foo = tableInNestedNS.Foo()
+
+    # TableInNestedNST
+    def Pack(self, builder):
+        TableInNestedNSStart(builder)
+        TableInNestedNSAddFoo(builder, self.foo)
+        tableInNestedNS = TableInNestedNSEnd(builder)
+        return tableInNestedNS