LSIF: Accept multiply defined documents in LSIF uploads

Created by: efritz

This allows the input from lsif-tsc to be accepted by the server. This indexer, in particular, will index dependent projects as a pre-step before indexing the target project. This actually creates that can be thought of as multiple separate dumps in the same index file. Multiple references to a file between these projects can cause a document to become defined in more than one of these mini-indexes.

There should be no change in behavior here, and this should be well-behaved on new inputs with multiple documents. There may be additional information that's not used or doubly-queried, but we can prune these in a future PR if we find it necessary.