SI-8170 Fix regression in TypeRef#transform w. PolyTypes

Regressed in SI-8046 / edc9edb7, by my hand.

At the time, I noticed the problem: transform wasn't accounting
for the potential Poly-Type-ness of its argument, and this would
lead to under-substituted types. The commit comment of edc9edb7
shows an example.

But the remedy wasn't the right one. The root problem is
that a TypeMap over a PolyType can return one with cloned
type parameter symbols, which means we've lose the ability
to substitute the type arguments into the result.

This commit detects up front whether the type-under-transform
is a PolyType with the current TypeRef's type parameters, and
just runs the `asSeenFrom` over its result type.

Review by @adriaanm