cmd/compile: change exporter.pos to take src.XPos instead of *Node
cmd/compile: change exporter.pos to take src.XPos instead of *Node

This is to allow followup refactorings that will replace
Field.Nname.Pos with Field.Pos.

Passes toolstash-check.

Change-Id: I1060b6a37c60273892f7af5369809057cff61881
Reviewed-on: https://go-review.googlesource.com/108215
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

CAFxX/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,josharian/go,josharian/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,nawawi/go,josharian/go,akutz/go,josharian/go,CAFxX/go,golang/go,josharian/go,akutz/go,cloudflare/go,josharian/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,josharian/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,josharian/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,CAFxX/go