The old code simply printed the position of the line directive in
square brackets for a position modified by a line directive. Now
we print the corresponding actual source file position instead.

Fixes #19392.

Change-Id: I933f3e435d03a6ee8269df36ae35f9202b7b2e76
Reviewed-on: https://go-review.googlesource.com/37932
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>