cmd/compile/internal/syntax: remove unused field in (scanner) source

The source.offs field was intended for computing line offsets which
may allow a tiny optimization (see TODO in source.go). We haven't
done the optimization, so for now just remove the field to avoid
confusion. It's trivially added if needed.

While at it, also:

- Fix comment for ungetr2.
- Make sure sentinel is present even if reading from the io.Reader failed.

Change-Id: Ib056c6478030b3fe5fec29045362c8161ff3d19e
Reviewed-on: https://go-review.googlesource.com/c/152763


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>