go/ast: trailing func comments mishandled in CommentMap

Working example: https://play.golang.org/p/TlsDxVbGow

Prints: g: "trailing comment in f\n"

Seems like the correct output should either be nothing (if it is associated with _ = x), or f: "trailing comment in f\n".

What's currently happening is that the "_ = x" in func f is the most recent "important" node when we encounter the comment. Since the comment isn't on the line immediately following "_ = x", it doesn't get associated with it. We don't look further up the "important nodes" stack to notice that it's within the bounds of f. Instead, we just give up and associate it with g.

/cc @griesemer