[dev.typeparams] cmd/compile/internal/types2: provide valid signature in...

[dev.typeparams] cmd/compile/internal/types2: provide valid signature in errors involving method expressions

This is an adjusted port of a similar fix in https://golang.org/cl/324733.

Fixes #46583.

Change-Id: Ica1410e4de561e64e58b753e3da04b32156cbaf6
Reviewed-on: https://go-review.googlesource.com/c/go/+/325369


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>