internal/gocore: ignore non-ClassExprLoc locations when reading locations

viewcore only knows how to handle ExprLoc-encoded location information.
As of Go 1.11, we now encode locals with ClassLocListPtr.

This CL just skips location list info. That's fine for globals (they
are still ExprLoc-encoded), but for locals we'll need a CL to make
gocore understand location lists. This CL just gets us to a point
where viewcore doesn't crash.

Change-Id: Ibde9ebcb130bc7e9141e7b6162e1500ff8b852e8
Reviewed-on: https://go-review.googlesource.com/127118
Reviewed-by: Heschi Kreinick <heschi@google.com>