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.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)