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.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