cmd/compile: ensure generic function is loaded when it needs to be re-exported

In the case where we need to re-export a generic function/method from
another package in the export data of the current package, make sure it
is loaded before trying to write it out.

Fixed #49667

Change-Id: I177754bb762689f34cf5c8ad246d43f1cdbbf195
Reviewed-on: https://go-review.googlesource.com/c/go/+/365837


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>