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".