[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.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/idl_gen_cpp.cpp b/src/idl_gen_cpp.cpp
index 12e43375..f74abbf8 100644
--- a/src/idl_gen_cpp.cpp
+++ b/src/idl_gen_cpp.cpp
@@ -1837,6 +1837,9 @@ class CppGenerator : public BaseGenerator {
       code_ += "  typedef {{NATIVE_NAME}} NativeTableType;";
     }
     code_ += "  typedef {{STRUCT_NAME}}Builder Builder;";
+    if (opts_.g_cpp_std >= cpp::CPP_STD_17) {
+      code_ += "  struct Traits;";
+    }
     if (opts_.mini_reflect != IDLOptions::kNone) {
       code_ +=
           "  static const flatbuffers::TypeTable *MiniReflectTypeTable() {";
@@ -2198,6 +2201,16 @@ class CppGenerator : public BaseGenerator {
     code_ += "}";
     code_ += "";
 
+    // Definition for type traits for this table type. This allows querying var-
+    // ious compile-time traits of the table.
+    if (opts_.g_cpp_std >= cpp::CPP_STD_17) {
+      code_ += "struct {{STRUCT_NAME}}::Traits {";
+      code_ += "  using type = {{STRUCT_NAME}};";
+      code_ += "  static auto constexpr Create = Create{{STRUCT_NAME}};";
+      code_ += "};";
+      code_ += "";
+    }
+
     // Generate a CreateXDirect function with vector types as parameters
     if (has_string_or_vector_fields) {
       code_ +=