go/types, types2: pass the seen map through _TypeSet.IsComparable

While checking comparability of type parameters, we recurse through
_TypeSet.IsComparable, but do not pass the cycle-tracking seen map,
resulting in infinite recursion in some cases.

Refactor to pass the seen map through this recursion.

Fixes #50782

Change-Id: I2c2bcfed3398c11eb9aa0c871da59e348bfba5f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/380504


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>