Holds if GVNs t1 and t2 can be unified. That is, it is possible to
replace all type parameters in t1 and t2 with some GVNs (possibly
type parameters themselves) to make the two substituted terms equal.
Import path
import semmle.code.csharp.Unificationpredicate unifiable(ConstructedGvnType t1, ConstructedGvnType t2)