Replace hard-coded values for ITypeTag with generator.
Replace hard-coded values for ITypeTag with generator.

This is part of the solution for dealing with the incompatible change
introduced in this enum in gobject-introspection 0.9.1.

mvz/gir_ffi,jcupitt/gir_ffi,jcupitt/gir_ffi,mvz/gir_ffi,mvz/gir_ffi