Parser doesn't attach some compiler directives to anything in the tree.
We have to explicitely retain them in the generated code. This change,
makes cover explicitely print out any compiler directive that wasn't
handled in the ast.Visitor.

Fixes #18285.

Change-Id: Ib60f253815e92d7fc85051a7f663a61116e40a91
Reviewed-on: https://go-review.googlesource.com/34563
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>