When using //line directives and unformatted code it is possible for
positions to repeat. Increment the final column position to avoid that.

Fixes #27350

Change-Id: I2faccc31360075e9814d4a024b0f98b117f8ce97
Reviewed-on: https://go-review.googlesource.com/c/153061
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Rob Pike <r@golang.org>