cmd/compile: fix method expressions with anonymous receivers

cmd/compile: fix method expressions with anonymous receivers

Method expressions with anonymous receiver types like "struct { T }.m"
require wrapper functions, which we weren't always creating. This in
turn resulted in linker errors.

This CL ensures that we generate wrapper functions for any anonymous
receiver types used in a method expression.

Fixes #22444.

Change-Id: Ia8ac27f238c2898965e57b82a91d959792d2ddd4
Reviewed-on: https://go-review.googlesource.com/105044
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>