[dev.inline] cmd/compile/internal/syntax: introduce general position info for nodes

Reviewed in and cherry-picked from https://go-review.googlesource.com/#/c/33758/.
Minor adjustments in noder.go to fix merge.

Change-Id: Ibe429e327c7f8554f8ac205c61ce3738013aed98
Reviewed-on: https://go-review.googlesource.com/34231


Reviewed-by: Matthew Dempsky <mdempsky@google.com>