Fixed an error that occurred in atomic mode. cover adds a global
variable declaration that forces sync/atomic to be used. fixDirectives
was confused by this declaration since it has an invalid
position. These declarations are now skipped.

Fixes #22309

Change-Id: I84f5fec13ef847fca35ad49f7704fb93b60503e0
Reviewed-on: https://go-review.googlesource.com/71351
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>