[release-branch.go1.16] cmd/link: handle types as converted to interface when dynlink

When using plugins, a type (whose value) may be pass to a plugin
and get converted to interface there, or vice versa. We need to
treat the type as potentially converted to interface, and retain
its methods.

Updates #44586.
Fixes #44638.

Change-Id: I80dd35e68baedaa852a317543ccd78d94628d13b
Reviewed-on: https://go-review.googlesource.com/c/go/+/296709


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
(cherry picked from commit a655208c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/296910