go/ast, go/printer: recognize export and extern line directives

go/ast, go/printer: recognize export and extern line directives

Now that gofmt is reformatting these, we can't get away with
not knowing about directives such as //export and //extern (for gccgo).
Otherwise "//export foo" and "//extern foo" turn into "// export foo",
and "// extern foo", which are completely different meanings.

For #51082.

Change-Id: Id0970331fa0b52ab5fa621631b5fa460767068bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/399734
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>