[meta] TDictionary::GetDictionary() prefer class over typedef:

For `std::string`, we have both a TClass and a TDataType (as it is a typedef).
We should prefer the TClass (which fixes issue #7169), but not at the cost
of calling `TClass::GetClass()` first: that is just too slow. So instead,
check for "is it a datatype, not numeric, do we have a TClass?" which will
still prefer the TClass even if *also* a TDataType exists.

Provide documentation!