This PR carries over some of the `has_relation_to` logic for comparing a
typevar with itself. A typevar will specialize to the same type if it's
mentioned multiple times, so it is always assignable to and a subtype of
itself. (Note that typevars can only specialize to fully static types.)
This is also true when the typevar appears in a union on the right-hand
side, or in an intersection on the left-hand side. Similarly, a typevar
is always disjoint from its negation, so when a negated typevar appears
on the left-hand side, the constraint set is never satisfiable.
(Eventually this will allow us to remove the corresponding clauses from
`has_relation_to`, but that can't happen until more of #20093 lands.)
eda85f3c64