cmd/cover: add //line comment pointing to original file

cmd/cover: add //line comment pointing to original file

Now that cover does not modify the formatting of the original file
or add any newline characters, we can make it print a //line comment
pointing back at the original, and compiler errors and panics will
report accurate line numbers.

Fixes #6329.
Fixes #15757.

Change-Id: I7b0e386112c69beafe69e0d47c5f9e9abc87c0f5
Reviewed-on: https://go-review.googlesource.com/77151
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>