Index symbol occurences from synthetics

SemanticDB `TextDocument` has a "synthetics" section which is currently ignored by `lsif-java index-semanticdb`. The place where we index the "occurrences" section is here https://github.com/sourcegraph/lsif-java/blob/6aeae658cbd7624a59c08c270af22f5d7070b493/lsif-semanticdb/src/main/java/com/sourcegraph/lsif_semanticdb/LsifSemanticdb.java#L75

