[dev.inline] cmd/internal/src: make Pos implementation abstract

Adjust cmd/compile accordingly.

This will make it easier to replace the underlying implementation.

Change-Id: I33645850bb18c839b24785b6222a9e028617addb
Reviewed-on: https://go-review.googlesource.com/34133


Reviewed-by: David Lazar <lazard@golang.org>