[dev.inline] cmd/compile/internal/syntax: report byte offset rather then rune count for column value

This will only become user-visible if error messages show column information.
Per the discussion in #10324.

For #10324.

Change-Id: I5959c1655aba74bb1a22fdc261cd728ffcfa6912
Reviewed-on: https://go-review.googlesource.com/34244
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>