[SourceKit] Disable automatic rebuilding of dependent-ASTs
After modifying a file, don't rebuild the ASTs for every file that
(might) depend on it. This is very expensive if there are a lot of open
documents and we don't yet provide a way for clients to distinguish
documents that they want to update from any other documents that e.g.
are just open because they haven't been saved yet.
rdar://problem/34415818
