cmd/compile: mark plugin-exported types as used in interface

Plugin exports symbols as interfaces. Mark their types as used in
interfaces, so their methods will be kept alive by the linker.

Fixes #42579.

Change-Id: If1b5aacc21510c20c25f88bb131bca61db6f1d56
Reviewed-on: https://go-review.googlesource.com/c/go/+/269819


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>