go/ast: add missing handling for ListExpr in Walk
go/ast: add missing handling for ListExpr in Walk

Change-Id: I96170f1ca481b5fe21c85f99e877fa090e4dccc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/304452
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,golang/go