go/types: keep generic methods out of method sets

Method sets define the set of methods which can be called on a value
of a type, as well as the interfaces the type implements. Generic
methods cannot be used to implement interfaces, nor can a generic
method be called.

Change-Id: Ifba6c558c045a6d377025fe2558d8821274dbe86
Reviewed-on: https://go-review.googlesource.com/c/go/+/781264
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>