go/types, types2: fail interface inference against generic methods

When unifying an interface against a concrete type, interface inference
looks up each interface method on the concrete type and unifies the
signatures. If the concrete method is generic, the *Signature unify
case (which ignores type parameter lists) would unify the method's own
type parameter into the inference variable, leading to errors mentioning
a leaked P and recording the wrong type in Info.

A generic method never satisfies an interface method, so fail the
unification at the lookup site instead.

Change-Id: Id01eefcde492399cb94f4452f5627c4edbd22a1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/766160
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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