codeintel: Add ensureUnambiguousResultSets to lsif-validate
Created by: efritz
LSIF spec dictates that a resultSet form a chain and not a tree reachable from a range (so that fallback through the graph is deterministic). Let's validate that.
(This validation pass discovers an implementation issue in https://github.com/sourcegraph/lsif-go/pull/142)