CL 61111 disabled the writing of trivial.c in -n mode, which
made -n mode at least inconsistent with regular mode in
how it was testing for flags. We think that both were getting
the same answer, so avoid creating the file in both modes
to make sure.

If this CL turns out to be wrong, then when we revert it we
should make sure that the empty file is written even in -n mode,
because this check affects the command-line flags printed
by other commands in that mode.

Change-Id: I0a050bfc148fe5a9d430a153d7816b2821277f0d
Reviewed-on: https://go-review.googlesource.com/78115
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>