make flatbuffers::IsFieldPresent safer (#4988)

Give the vtable offset enum inside each table the name
"FlatBuffersVTableOffset" and base type voffset_t so it can be used as a
dependent type in IsFieldPresent. This makes that function slightly
safer since it prevents calling it with arbitrary, non-table types.
Now, the only way to use IsFieldPresent incorrectly is to create your
own type which does not inherit from flatbuffers::Table but has a
dependent voffset convertible type "FlatBuffersVTableOffset".

src/idl_gen_cpp.cpp

@@ -1710,7 +1710,7 @@ class CppGenerator : public BaseGenerator {
       // We need to add a trailing comma to all elements except the last one as
       // older versions of gcc complain about this.
       code_.SetValue("SEP", "");
-      code_ += "  enum {";
+      code_ += "  enum FlatBuffersVTableOffset FLATBUFFERS_VTABLE_UNDERLYING_TYPE {";
       for (auto it = struct_def.fields.vec.begin();
            it != struct_def.fields.vec.end(); ++it) {
         const auto &field = **it;