URLs come through SourceKit as custom <Link> elements which weren't
getting escaped as raw HTML, which would cause URLs with query
strings to get cut off because of ampersands.
rdar://problem/23394719
These elements were getting parsed but dropped on the floor.
Now, they'll be added to the Swift Markup AST if they are
present and emitted in the HTML.
rdar://problem/22369731
Swift SVN r31452