[dev.typeparams] go/types: use comparable bit rather than ==() method

[dev.typeparams] go/types: use comparable bit rather than ==() method

This is a port of CL 337354 to go/types, adjusted for the error
reporting API and to reposition a couple error messages in
issue47411.go2 (the go/types position is probably better).

A panic is also fixed in lookup.go when method lookup fails and static
== false. I'll send a fix for types2 in a separate CL.

For #47411

Change-Id: Icc48f03c3958695f581f10e8675c1f32434c424b
Reviewed-on: https://go-review.googlesource.com/c/go/+/339652
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>