cmd/compile/internal/syntax: implement Pos.FileBase method (cleanup)

cmd/compile/internal/syntax: implement Pos.FileBase method (cleanup)

Factor out file base computation into a method.

Change-Id: Ia6de100459b6df2919f2320872890320aa88866d
Reviewed-on: https://go-review.googlesource.com/c/go/+/586156
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>