Change dump file names to group them alphabetically in directory
listings, in pass run order.

Change-Id: I8070578a5b4a3a7983dcc527ea1cfdb10a6d7d24
Reviewed-on: https://go-review.googlesource.com/83958Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>