This resulted in a codegen error in C++ mode, because the generic
types were not defined in modules where calls requiring downcasts
were used (generating a downcast forces the inclusion of the full
definition of the involved types).
A more efficient implementation is possible by restoring the old
lifting ot tyGenericInvocation to tyGenericInst in liftTypeParam,
but this fix will suffice for now.
fixes#5087fixes#5602fixes#5641fixes#5570