[C++17] Add Traits class for Tables and Factory function within it. (#5678)

* Include flattests_cpp17 in unit tests when C++17 build is enabled.

* [C++17] Generate generic table factory function.

1. For each table, generate a convenient free-standing factory
   function that allows creating the table in a generic way by
   specifying only the type.  This is the first change in a series
   of changes to make Flatbuffers generated C++ code more friendly
   to code bases that make use of C++ template metaprogramming
   techniques to manage the serialization process.  Example:

     Before :(

       // The name of the Flatbuffers type (and namespace) must
       // be hard-coded when writing the factory function.
       auto monster = MyGame::Example::CreateMonster(fbb, ...);

     After  :)

       using type_to_create = MyGame::Example::Monster;
       // No namespace needed on CreateByTagType.
       auto monster = CreateByTagType((type_to_create*)nullptr,
                                      fbb, ...);

   This feature requires building with C++14 or greater, and thus
   it is guarded behind --cpp-std >= c++17 in the flatbuffers C++
   generator.

2. Fix a CMake bug to include C++17 unit tests in test suite.

* [C++17] Replace standalone variadic factory function with type_traits.

Add a `type_traits` to each table class.  This `type_traits` can be
populated with various compile-time info about the table.  Initially,
we have the Create* function and type, but is extensible in the future.

* Remove empty line and fix stale comments.

* Rename type_traits to Traits and move fwd declaration.

* Fix parameter evaluation order issue and use lambda for scope.

tests/cpp17/test_cpp17.cpp

@@ -36,8 +36,31 @@ namespace cpp11 {
 #include "../monster_test_generated.h"
 }  // namespace cpp11
 
+void CreateTableByTypeTest() {
+  flatbuffers::FlatBufferBuilder builder;
+
+  // We will create an object of this type using only the type.
+  using type_to_create_t = cpp17::MyGame::Example::Stat;
+
+  [&builder] {
+    auto id_str = builder.CreateString("my_id");
+    auto table = type_to_create_t::Traits::Create(builder, id_str, 42, 7);
+    // Be sure that the correct return type was inferred.
+    static_assert(
+        std::is_same_v<decltype(table), flatbuffers::Offset<type_to_create_t>>);
+    builder.Finish(table);
+  }();
+
+  // Access it.
+  auto stat =
+      flatbuffers::GetRoot<type_to_create_t>(builder.GetBufferPointer());
+  TEST_EQ_STR(stat->id()->c_str(), "my_id");
+  TEST_EQ(stat->val(), 42);
+  TEST_EQ(stat->count(), 7);
+}
+
 int FlatBufferCpp17Tests() {
-  TEST_ASSERT(true);
+  CreateTableByTypeTest();
   return 0;
 }